fix(compaction): diagnose missing generation before preparation (#645)

A map-required compaction preparation for a seat whose current occupant has no generation could never succeed: the map it waits for cannot be matched to a null generation, so the request ran out its full budget and failed. Preparation now checks both existing generation resolvers first and, when neither yields an identity, stops immediately with a reason that names the condition and the supported recovery: run rig reconcile-session '<session>' --no-launch, then start a new rig compact attempt. No preparation prompt is sent in that case.

Nothing else changes: skip-map still works with no generation, an active attempt still cannot be taken over, the 120 s manual and 180 s HTTP budgets are unchanged, and cancel, expiry, disable, late-map and stale-generation handling stay as they were. The fix never invents an identity, reconciles a seat on its own, or rearms a stopped attempt; only a new deliberate attempt after reconciliation freezes a new generation and map.

Refs #610