TARIFF-18: price_taxes_eur_kwh stops shipping at half the real figure

`price_taxes_eur_kwh` shipped at 0.06 EUR/kWh with no recorded provenance -
no card, no date, no component list - and roughly HALF the true Flemish
all-in non-energy figure. It now ships at 0.11736, and the rows it is made
of are in the code rather than in this message:

  Fluvius distribution (Antwerpen)              5,35   c EUR/kWh
  Bijzondere accijns, "Alle verbruik"           4,876
  Bijdrage groene stroom en WKK Vlaanderen      1,51
  Energiebijdrage (0 EUR/MWh since 2026-08-01)  0
                                               ------
                                               11,736 c EUR/kWh = 0.11736

Eneco residential card for Flanders, September 2026, incl. 6 % VAT.
`app/price.py`'s FLEMISH_TAXES_COMPONENTS_EUR_KWH carries the rows and
test_price.py section 1b re-adds them - and also compares the total against
the card's own figures typed in the test file, so an edit that moved the
rows and the default together is still caught.

WHAT THIS IS NOT. It is not an arbitrage-correctness fix and it cannot be.
This scalar is a LEVEL component: an additive constant cancels exactly in
`max - min` and leaves the argmin where it was. TARIFF-17 shipped that proof
and it is unchanged - test_price.py 16b drives six flat scalars spanning and
exceeding the whole per-DSO range and gets the identical cheapest hour and
the identical spread to the last digit. DOCS and the CHANGELOG say so in
those words.

WHAT IT IS. A display and guard fix:

  * every number an operator reads, which at half the true value made the
    add-on look considerably better than it is; and
  * §17's negative-price guard in arbitrage.py - the one rung that tests an
    ABSOLUTE all-in price rather than a difference. It fires when
    `spot < -(markup + taxes)`, so its boundary moves -0.09 -> -0.14736
    EUR/kWh of spot. test_arbitrage.py section 2c drives both sides through
    the shipped decide() and pins the boundary from below as well.

The default is Antwerpen's real row, near the LOW end of the 0.117-0.131
per-DSO band, and that is deliberate: the error directions are not
symmetric. Too high carries a genuinely negative hour above zero and the
add-on exports into paid-for disposal - §17's own case. Too low only
declines a discharge that would have paid.

TARIFF-17's INTERACTION, WHICH HAD TO BE DECIDED. Nothing in that rung
hardcoded 0.06; it read PRICE_OPTION_DEFAULTS, so moving the default moves
what it refuses. The Supervisor sets `App.options = deepmerge(config.yaml
defaults, stored options)`, so a saved blob carries the key whether it was
typed or not - every stored 0.06 became "non-default" with nobody editing
anything, and a working Walloon or Brussels site with a taxes table would
have met SystemExit(78) on the upgrade over an inert number. price.py now
carries SUPERSEDED_TAXES_DEFAULTS_EUR_KWH = (0.06,) and validate.py reads a
superseded default as untouched too. The rung is NOT weakened: 0.1198 beside
a table is still refused by name, and the refusal now names both figures.

config.yaml and PRICE_OPTION_DEFAULTS moved together; CFG-03's tripwire
(test_price.py section 1, test_defaults_pin.py) stayed green by agreeing.
test_price.py section 0's fourth check was REWRITTEN rather than re-pointed:
it used to justify the file's own grading constants by observing that the
shipped pair was degenerate (0.06 is exactly twice 0.03). TARIFF-18 ends
that coincidence, so the sentence is no longer true and is not kept.

29/29 test files green. Mutation ledger on the PR.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwRLw7eKYj2byVdwB6kbjU

goodwe_controller/DOCS.md

@@ -1371,7 +1371,7 @@ does, and only after you have read why.
 |---|---|
 | `price_provider` | `off` (default) or one of the three plugins below |
 | `price_markup_eur_kwh` | Your supplier's margin, EUR/kWh. **Added only when the chosen plugin's feed does not already contain it** |
-| `price_taxes_eur_kwh` | Levies and VAT, EUR/kWh. Same rule |
+| `price_taxes_eur_kwh` | Distribution, levies and VAT, EUR/kWh. Same rule. Defaults to **0.11736** — a real Flemish figure, and **yours is probably different**: see *What `price_taxes_eur_kwh` is made of* below |
 | `export_price_eur_kwh` | The **injection** price. A separate scalar, not a series — see below |
 | `price_csv_path` | Where `manual_csv` reads its file |
 | `price_feed_url` | Override the plugin's default feed. Used **exactly as written** — the add-on appends nothing to it, including the date range, so an override gets whatever horizon that URL returns. Only a host the plugin itself declares is accepted |
@@ -1891,10 +1891,11 @@ size:
 | **ORDER** | **varies** by hour, block or day-type | **Yes, and worst.** It changes *which hour is cheapest* |
 
 Measured on the real arithmetic (`test_price.py` section 16b, printed on every
-run): with a flat taxes figure of 0.00, 0.0739, 0.1198, 0.1327 or 1.00 EUR/kWh,
-the same 24-hour feed is cheapest at **13:00 in all five cases** and its spread
-is **0.1500 EUR/kWh in all five cases**. That is the LEVEL bucket doing
-nothing, and it is not a rounding artefact — it is arithmetic.
+run): with a flat taxes figure of 0.00, 0.0739, 0.11736 (the shipped default),
+0.1198, 0.1327 or 1.00 EUR/kWh, the same 24-hour feed is cheapest at **13:00 in
+all six cases** and its spread is **0.1500 EUR/kWh in all six cases**. That is
+the LEVEL bucket doing nothing, and it is not a rounding artefact — it is
+arithmetic.
 
 Three consequences worth having in mind:
 
@@ -1925,6 +1926,83 @@ Three consequences worth having in mind:
 is the only ORDER component the add-on has. Anything added next belongs in one
 of the three, and saying which is part of adding it.
 
+### What `price_taxes_eur_kwh` is made of, and why yours is different (TARIFF-18)
+
+**It ships at `0.11736` EUR/kWh.** That is the all-in non-energy part of a
+Flemish residential bill — everything on your invoice that is *not* the energy
+itself — read off the Eneco residential card for Flanders, **September 2026**,
+**including 6 % VAT**:
+
+| component | c€/kWh |
+|---|---|
+| Fluvius distribution (**Antwerpen**) | 5,35 |
+| Bijzondere accijns, *"Alle verbruik"* — no volume bands | 4,876 |
+| Bijdrage groene stroom en WKK Vlaanderen | 1,51 |
+| Energiebijdrage | **0** |
+| **total** | **11,736 c€/kWh = 0.11736 €/kWh** |
+
+The rows are in the code, in `app/price.py`'s
+`FLEMISH_TAXES_COMPONENTS_EUR_KWH`, and `test_price.py` section 1b re-adds them
+and refuses a default that is not their sum. It replaces a shipped `0.06`,
+which was roughly **half** the true figure and had no recorded provenance at
+all.
+
+⚠️ **`energiebijdrage` is zero and that is not a mistake.** *"Vanaf 1 augustus
+2026 bedraagt de energiebijdrage 0 EUR/MWh"* — it was merged into the
+bijzondere accijns. If you are re-deriving this total from a guide written
+before that date you will find a separate energiebijdrage line; adding it would
+count it twice. It is carried as a **zero row** rather than left out for
+exactly that reason.
+
+⚠️ **THIS IS A PER-DSO FIGURE AND YOURS IS PROBABLY NOT 0.11736.** Only the
+distribution row moves. Across the eight Fluvius DSOs it runs **5,28 c€/kWh**
+(Midden-Vlaanderen) to **6,70** (West), so the Flemish all-in total runs about
+**0.117 to 0.131 €/kWh** — a 1,42 c€/kWh spread, which on 4 000 kWh a year is
+about €57. The other two rows are Flanders-wide and do not move. **Look up your
+own distribution row, add the same accijns and groenestroombijdrage, and type
+the total.**
+
+⚠️ **A WALLOON OR BRUSSELS SITE SHOULD NOT USE THIS SCALAR AT ALL.** Their
+distribution tariff has a **day and a night band** — `Enkelvoudig | Dag |
+Nacht | Uitsl. nacht` on the same card, where Fluvius prints a single rate —
+and a flat figure flattens exactly the component that is anti-correlated with
+what the battery wants to do. Use **`price_taxes_profile`** (the next section);
+it is the only ORDER-bucket component the add-on has.
+
+⚠️ **THIS IS A DISPLAY AND GUARD FIX. IT DOES NOT IMPROVE ARBITRAGE, AND IT
+CANNOT.** `price_taxes_eur_kwh` is a **LEVEL** component — added identically to
+every hour — so it **cancels exactly** in `max − min` and leaves the argmin
+where it was. It does not change which hour is cheapest, it does not change the
+spread, and it does not make any trade happen that did not happen before. See
+*The three buckets* above, where six flat scalars spanning and exceeding the
+whole per-DSO range give the identical cheapest hour and the identical spread
+to the last digit. What raising it **does** change is exactly two things:
+
+* **every figure an operator reads** — the all-in prices on the status page,
+  `custom_formula`'s startup preview and the recorded price history. At half
+  the true non-energy component they made the add-on look considerably better
+  than it is.
+* **§17's negative-price guard** (`arbitrage.py`), which is the one rung that
+  tests an **absolute** all-in price rather than a difference. The guard fires
+  when `spot < −(markup + taxes)`, so its boundary moved from **−0.09000** to
+  **−0.14736 €/kWh** of spot. A spot hour at −0.11868 €/kWh used to read
+  **−0.02868 €/kWh** all-in and the discharge was refused as paid-for disposal;
+  at the real tax level it reads **+0.02868** and the discharge is allowed,
+  because the operator really does pay those levies and that hour really was
+  profitable (`test_arbitrage.py` section 2c drives both, and pins the boundary
+  from below as well).
+
+⚠️ **Which is why the default sits near the LOW end of the per-DSO band rather
+than in the middle of it.** The two error directions are not symmetric. Set too
+**high**, the figure carries a genuinely negative hour up above zero and the
+add-on exports into paid-for disposal — §17's own regression case. Set too
+**low**, it only declines a discharge that would have paid. Where one default
+has to stand in for eight DSOs, it belongs at the bottom.
+
+**If you set this yourself, nothing above affects you.** A site with its own
+`price_taxes_eur_kwh` keeps it; the default only answers where the option is
+absent from the stored options blob.
+
 ### Levies and VAT by time of day (`price_taxes_profile`)
 
 **Leave this empty unless your network tariff really does have a day and a
@@ -1979,8 +2057,12 @@ second set of 23- and 25-hour-day bugs.
 - ⚠️ **Set this *or* `price_taxes_eur_kwh`, not both.** The table wins when it
   is set, so a scalar left at a non-default value beside it would be a number
   you typed that decides nothing. That is refused at startup rather than
-  silently resolved; leave `price_taxes_eur_kwh` at its shipped `0.06` when you
-  fill this in.
+  silently resolved; leave `price_taxes_eur_kwh` at its shipped `0.11736` when
+  you fill this in. ⚠️ **A scalar still sitting on the superseded `0.06` is
+  also accepted**, and deliberately so — TARIFF-18 moved that default, and your
+  stored options blob carries the old number whether you typed it or not, so
+  refusing it would have stopped working Walloon and Brussels sites on an
+  upgrade over a figure the table overrides anyway.
 - A **malformed table is refused at startup**, not at the next fetch. It is
   read on the ~30 s planning path, and a table that first fails at 03:00 is a
   site that stops pricing at 03:00 — which §11.4 turns into a quiet fallback