TEL-04 review: unshadow the helper, validate config at startup, and record

what the rig proved about the age sensor

Review findings 1, 3, 5 and 6. Finding 2 is deliberately untouched - it is
its own ticket.

3. `built` was rebound at test_p1.py:816 by `built = build_source(...)`,
   silently disarming the build() wrapper for anything appended below it.
   Renamed to `sel`. Reproduced the reviewer's failure before fixing:
   appending a check that calls built() after that line gives
   `TypeError: 'HaSignedSource' object is not callable` and aborts at 163 of
   180; with the rename the same probe reaches 180 and passes.

5. DOCS.md now states the "length must equal meter_phases" constraint that
   config.yaml already carried, plus what leaving the list empty actually
   costs: on the surveyed reading the phases carry 2769 W of import while the
   connection nets 187 W, so the tariff quantity is understated ~15x.

6. build_source now checks the ha_signed wiring once at startup instead of
   once per telegram: a blank p1_net_entity, or a phase list whose length
   disagrees with meter_phases, logs an error and disables ingestion. Both
   otherwise fail in the single way indistinguishable from a healthy source
   nobody has fed yet - no samples, a climbing age, the watchdog holding the
   battery at 0 W, and nothing in the log.

1. THE AGE SENSOR. Measured on the ENV-01 rig against the real HomeWizard
   integration, meter frozen via hwsim's `?fault=freeze` seam (cleared in a
   finally:, rig verified restored):

     - websocket state_changed for the meter over 70 s : 0
     - last_reported advanced (REST serialiser)        : no
     - last_reported advanced (websocket serialiser)   : no
     - subscribe_events(state_reported)                : rejected,
       "Event filter is required for event state_reported"

   So Home Assistant exposes NO arrival signal for a repeated reading, and
   the proposed fix - stamp from last_reported via subscribe_entities - is
   not available. subscribe_entities listens only to EVENT_STATE_CHANGED, and
   as_compressed_state carries no last_reported at all.

   The age is therefore "time since the value changed", which on ha_dsmr is
   mostly harmless (a telegram moves several entities) and on ha_signed is
   not: one entity means a healthy meter under a flat load is
   indistinguishable from a dead one. Recorded loudly in DOCS.md, in the
   HaSignedSource docstring and in the CHANGELOG, with the measured 42.2 s
   and 97.0 s gaps from our own capture.

   meter_max_age_s is deliberately NOT widened. The two conditions produce an
   identical signal, so a larger number does not separate them - it only
   chooses which of the two errors you get, and it would disarm the watchdog
   for a genuinely dead meter as well. The honest fix is an arrival stamp the
   meter itself provides.

test_p1.py: 174 -> 179 checks, all green. Other three suites unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Du77usMj8XNKNFZGmUiWDa

goodwe_controller/CHANGELOG.md

@@ -20,6 +20,20 @@ across the three sources.
 
 Still defaults to `off`; an existing install is unaffected until it opts in.
 
+⚠️ **`sensor.p1_sample_age_s` is published on `ha_signed`, but must not yet be
+thresholded by the ESP32 stale-input watchdog.** On the HA WebSocket paths the
+age is stamped from `state_changed`, so it measures time since the value
+*changed*, not since the meter *reported* - and Home Assistant exposes no
+arrival signal for a repeated reading (no `state_changed`, no `last_reported`
+movement on either serialiser, and `state_reported` is not subscribable over
+the WebSocket). Measured on the ENV-01 rig against the real HomeWizard
+integration. `ha_dsmr` mostly escapes it because a telegram moves several
+entities at once; `ha_signed` has one, so a healthy meter under a flat load is
+indistinguishable from a dead one. Our own capture has the house meter going
+42.2 s and 97.0 s between changes. Raising `meter_max_age_s` does not fix that,
+it only chooses which error you get; the fix is an arrival stamp from the meter
+itself and is a separate ticket. Full detail in DOCS.md.
+
 ## 0.3.0
 
 **SAFETY-04.** The control law's integrator is now an explicit accumulator,