fix: recover last trip distance when the board wipes it on remote power-cycle

[jmdevita]

Problem

The board reports per-trip distance via DP5 mileage_once, but the board wipes DP5 to 0 when the remote is power-cycled — e.g. turning it off then back on to "sync" once you're home in range. That blanked a just-completed ride's distance on the dashboard (shows 0.00 mi).

Fix

Trip-distance recovery now lives in the coordinator, the only place with access to both the previous and current read. A new EboardSnapshot.trip_distance_km10 carries an effective last-trip value computed each read:

• DP5 nonzero → trust it as-is.
• DP5 == 0 and the cumulative odometer (DP12) advanced since the last read → a ride happened between reads, so use the odometer delta.
• DP5 == 0 and the odometer is flat → a reconnect/sync with no new riding; keep the last known trip instead of overwriting it with 0.

The trip_distance sensor now reads the recovered value instead of DP5 directly.

Notes / known limits

• The first read immediately after an HA restart has no in-memory previous snapshot, so a sync read at that exact moment can still fall through to 0 (self-corrects on the next ride). Documented, deliberately not addressed — fixing it would mean persisting coordinator state backwards from the entity.
• Last trip time (DP6 ridetime_once) is not recoverable the same way: there is no cumulative ride-time counter to derive a delta from, and the board reports DP6 inconsistently. Left as-is.

Testing

• ruff check custom_components — clean
• pytest -q — 28 passed (pure suite, no hardware/HA)

Bumps manifest to 0.3.1.