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/app/custom_formula.py

@@ -520,13 +520,16 @@ def implausible_all_in(spec: FormulaSpec, taxes_eur_kwh: float):
 
     ⚠️ THE LOW END IS GRADED ON THE ENERGY COMPONENT AND THE HIGH END ON THE
     ALL-IN FIGURE, AND THAT ASYMMETRY IS THE CHECK RATHER THAN AN OVERSIGHT.
-    `price_taxes_eur_kwh` ships at 0.06, which is three times the low bound -
-    so a card read a HUNDRED times too small comes out at 0.0016 EUR/kWh of
-    energy, is carried over the floor by the flat tax addend alone, and passes
-    a band applied to the all-in total. The hundredfold error is precisely
-    what this exists to catch, so the floor is applied where it can still see
-    it. The ceiling is applied to the all-in figure because that is the number
-    the site is really billed at, and taxes only push it up.
+    `price_taxes_eur_kwh` ships at 0.11736 (TARIFF-18; it was 0.06 before, and
+    the argument was already decisive at that figure), which is nearly SIX
+    times the low bound - so a card read a HUNDRED times too small comes out at
+    0.0016 EUR/kWh of energy, is carried over the floor by the flat tax addend
+    alone, and passes a band applied to the all-in total. The hundredfold error
+    is precisely what this exists to catch, so the floor is applied where it
+    can still see it. ⚠️ Raising the default made this trap WIDER, not
+    narrower: the addend that does the carrying got bigger. The ceiling is
+    applied to the all-in figure because that is the number the site is really
+    billed at, and taxes only push it up.
 
     ⚠️ AND THE LIMIT, WITH THE CLAIM. This grades the CARD'S OWN POINT - one
     price, at the monthly index the card assumed - and not every hour of every