feat(config): wire live phase-aware grid-charge safety entities (issue #831, Part 1)

[woopstar]

Summary

Part 1 of a 3-PR plan to build a live per-phase Huawei grid-charge safety limiter (issue #831 — port of the safety-relevant half of Ambilights/hsem-ambilights#35, Huawei-only, no PowMr).

Why: the MILP plans grid-charging against a forecast, but between solve time and the hardware write, another appliance on the same phase can spike load. Without a live check immediately before the write, HSEM can command Huawei to charge at a rate that pushes a phase over the main fuse rating. The fork author hit this on real hardware; this PR starts building the same protection for this repo's Huawei-only control path.

This PR is foundation-only — no behavior change. The feature is off by default (hsem_phase_aware_charging_enabled = False) until Part 2 wires the actual limiter logic into the applier.

What's in this PR

• New config toggle hsem_phase_aware_charging_enabled (power step), default False.
• New entity picker hsem_huawei_solar_batteries_grid_charge_maximum_power (huawei_solar step) — the future limiter's write target. Maps to number.batteries_grid_charge_maximum_power, which already exists per docs/huawei_entities.md but was unwired.
• New entity pickers hsem_huawei_solar_power_meter_phase_{a,b,c}_active_power (power step) — live per-phase telemetry inputs. Map to sensor.power_meter_phase_{a,b,c}_active_power, also already documented but unwired.
• SensorConfig / LiveState fields for all four entities, read by config_reader.py / state_collector.py.
• LiveState.grid_phase_power_w: tuple[float | None, float | None, float | None] — fails closed to None per phase when a reading is unavailable/invalid, so the future limiter (Part 2) can't guess a missing phase.
• Diagnostics: new fields exposed via extra_state_attributes on the working-mode sensor (phase_aware_charging_enabled, grid_phase_power_w, huawei_batteries_grid_charge_max_power_w, plus entity-ID attributes under extended_attributes).
• Docs: docs/huawei_entities.md (marks both entities as used), docs/config-flow-reference.md, docs/sensors-reference.md.
• Translations: labels + descriptions added to both config and options steps in translations/en.json.

Files changed

• custom_components/hsem/const.py
• custom_components/hsem/custom_sensors/config_reader.py
• custom_components/hsem/custom_sensors/state_collector.py
• custom_components/hsem/custom_sensors/working_mode_sensor.py
• custom_components/hsem/flows/huawei_solar.py
• custom_components/hsem/flows/power.py
• custom_components/hsem/models/live_state.py
• custom_components/hsem/models/sensor_config.py
• custom_components/hsem/translations/en.json
• docs/config-flow-reference.md, docs/huawei_entities.md, docs/sensors-reference.md
• tests/sensors/test_state_collector.py, tests/test_config_validation.py

Delivery plan (tracked in issue #831)

• Part 1 (this PR): foundation wiring — entities configurable, off by default, no behavior change.
• Part 2: live phase limiter core — utils/phase_power.py gets compute_phase_charge_limits(); new custom_sensors/phase_charge_limiter.py computes a safe grid-charge cap from live phase telemetry; wired into applier.py so Huawei's grid-charge cap is actually written per-cycle when the toggle is on.
• Part 3: feedback-free floor + 45s fail-closed transition safety (the original feat(applier): live per-phase Huawei grid-charge safety limiter (Huawei-only port of Ambilights/hsem-ambilights#35 Stage 2) #831 ask) — prevents oscillation from a delayed battery-power echo during a verified downward cap change, and fail-closes to 0 W if telemetry doesn't settle within 45 seconds.

Tests

• tests/test_config_validation.py: schema round-trip for both new power step fields and the new huawei_solar step field; validation of the phase entity pickers (entity-not-found case); defaults-to-disabled case.
• tests/sensors/test_state_collector.py: build_sensor_config reads/defaults for phase_aware_charging_enabled, huawei_solar_batteries_grid_charge_maximum_power, and the three phase entities (including the vol.UNDEFINED → None normalization case, matching the existing _optional_entity() pattern used for other optional Huawei entities).

Test results

• ./scripts/quality.sh lint — pass (ruff format + check clean)
• ./scripts/quality.sh typing — pass (0 mypy errors)
• ./scripts/quality.sh quality — pass (0 pyright/vulture errors)
• ./scripts/quality.sh test — 2855 passed (full suite, including the new tests)

Known limitations

This PR only wires configuration and live-state plumbing. Enabling the toggle today has no runtime effect — Part 2 is required before the limiter actually computes and writes anything. This is intentional: it lets the config surface, translations, and data model land and be reviewed independently of the (larger, more safety-critical) limiter logic.

Part of #831.