Protect PHEV charging session from sync-down corruption

[renaudallard]

During PHEV AC charging, stale batteryManagement.header values could trigger the sync-down path in update_bmw_soc(), resetting anchor_soc and total_energy_kwh to zero. This caused two production issues:

1. SOC flicker: stale header (e.g. 5%) in same message as fresh charging.level (45%) briefly drops prediction before sync-up recovers — session data was destroyed in the interim.

2. Session discarded: at charge end, header arrives before status=NOCHARGING. Sync-down resets anchor to header value and energy to 0. Pending session then has soc_gain=0 and is discarded, losing the full charging session for learning.

Fix: when is_charging is True, the PHEV sync-down now only updates display values (_last_predicted_soc, session.last_predicted_soc) but preserves session.anchor_soc, total_energy_kwh, and last_energy_update. When not charging, full reset is preserved (no change).

Verified paths:

• BEV unaffected: entire block gated by _is_phev (BEVs take the elif-is_charging path which already has sync-up-only protection)
• Bug 1 trace: header=5 sets display to 5, session data preserved; charging.level=45 then syncs up cleanly. Entity never sees flicker (same async context, debounce fires after both processed)
• Bug 2 trace: header=56 with predicted=58 syncs display down but keeps anchor_soc=3 and total_energy_kwh=8.5; end_session reads soc_gain=56-3=53% with real energy — session preserved for learning
• Monotonicity guard: after sync-down sets session.last_predicted_soc low, next get_predicted_soc computes anchor+energy and max() picks the real prediction — display self-corrects on next tick
• Not-charging: full reset unchanged (anchor + energy zeroed)
• PHEV detection: i4 eDrive40 (BEV) safe — _is_metadata_bev override prevents misclassification even with erroneous fuel descriptors

[renaudallard]

Should solve #242