Entity cleanup, rain-forecast device_class fix, and PM ET0 / nowcast diagnostics

[kmich]

Six independent fixes/improvements verified against a live v2.8.1 instance on 2026-09-26. Each section below can be reviewed and reverted independently even though they ship as one PR. Checked open issues/PRs before opening this (twice: 2026-09-26 at task start and again just before opening) - nothing overlapping.

1. Remove orphaned local-forecaster entities

switch.ws_enable_local_forecaster and number.ws_forecaster_learning_rate show up on some live instances as restored: true / unavailable. A full-tree grep (switch.py, number.py, const.py, coordinator.py, learning_state.py, __init__.py) confirms zero current references.

Note on provenance: these unique IDs don't appear anywhere in this repo's git history either (427 commits, not shallow) - not even in 5cefe8d ("remove v2.0 AI/LLM code from public release"), the commit that stripped the closest related feature (WSAIProviderSelect, AI refresh-hours/token-budget numbers). They're leftovers of a pre-release build of the still-unreleased v2.0 AI/local-forecaster branch that a few instances briefly ran. Confirmed dead either way.

Cleanup runs on every setup/reload (not tied to a config-version bump, since affected instances are already on the latest schema) and only touches these two unique IDs for the current config entry - it does not touch any other restored/unavailable entity a user may have legitimately disabled.

2. Fix device_class on the rain-expected sensor

WSRainExpected1h (binary_sensor.ws_rain_expected_1h) used BinarySensorDeviceClass.MOISTURE, which Home Assistant reserves for a current wetness/leak reading, not a forecast flag. No binary sensor device_class fits a forecast prediction, so it's now unset.

3. Make PM ET₀ absence diagnosable

sensor.ws_et0_penman_monteith is gated on enable_solar_forecast but silently never appeared when the sources config had no solar_radiation key. A Repairs issue (et0_pm_unavailable) now explains this and points at the fix (map a solar radiation sensor, or enable the new illuminance fallback from #4).

4. Add a lux -> W/m² fallback source for PM ET₀

New optional "illuminance fallback" toggle (Solar Forecast step of the config/options flow, CONF_ET0_ILLUMINANCE_FALLBACK): when no solar_radiation source is mapped but an illuminance sensor is, derives an approximate solar radiation using the standard ~0.0079 W/m² per lux daylight conversion factor. The resulting sensor.ws_et0_penman_monteith value is tagged with a radiation_source attribute (sensor vs illuminance_estimate) so downstream consumers know it's lower-confidence than a real pyranometer reading. The conversion's real-world error margin (roughly ±25%, varying with sky conditions and sun elevation) is documented in const.py and the README FAQ.

5. Make nowcast staleness visible

binary_sensor.ws_rain_expected_1h depends on _async_fetch_nowcast() (Open-Meteo minutely_15). The coordinator's internal _nowcast_fetched_at is now exposed as nowcast_fetched_at / nowcast_stale attributes on the entity, and the nowcast cache is considered stale once older than 2x the fetch interval.

6. Add a local-only nowcast fallback

When the Open-Meteo nowcast has failed or gone stale (per #5's threshold), binary_sensor.ws_rain_expected_1h now falls back to a rain-expected signal derived from the existing local pressure-trend/Zambretti path (sensor.ws_zambretti_forecast's Z-number -> implied rain probability) instead of just going unavailable. This is coarser than the 15-minute NWP nowcast, so the entity's source attribute is tagged nowcast or local_fallback accordingly. The entity only goes explicitly unavailable when neither a fresh nowcast nor a Zambretti reading exists at all.

Testing

• tests/test_migration.py::TestOrphanedForecasterCleanup (item 1)
• tests/test_binary_sensor.py (items 2, 5, 6 - device_class, availability, source/staleness attributes)
• tests/test_coordinator.py::TestEt0PmUnavailableIssue, TestSolarRadiationForEt0 (items 3, 4)
• tests/test_coordinator.py::TestComputeNowcast (items 5, 6 - fresh/stale/never-fetched/no-fallback paths)
• Extracted the inline nowcast block in _compute() into _compute_nowcast() for direct unit testing.
• Full suite: 454 passed. ruff check / ruff format --check clean.

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