fix: guard replay reads against mid-hydration prefixes across backend respawns

[william0wang]

Problem

Phone wake-reconnect could enter a conversation whose replay ended in the middle; re-entering the same conversation replayed it whole. Root cause (diagnosed across bridge + client, source-verified against upstream): the backend's session/messages returns a PREFIX while a resumed session's store is still hydrating, and three bridge-side gaps let an unguarded plain read ship that prefix (or an empty read) to clients:

1. backendLoadedSessions stamps survived backend respawns (ensureBackend cleared hydration bookkeeping but not the stamps) — isBackendSessionLive stayed true for a process that never loaded the session, so session/load's alreadyLive check skipped the resume RPC.
2. A settle poll that exits "stable" inside a hydration gap never arms hydrationUnsettled — later plain reads had no protection at all.
3. runOneTurn's finally stamped backend-loaded unconditionally — a backend lost mid-turn with a FAILED recovery left a fresh stamp vouching for a session the respawned backend never loaded (ghost stamp, 5-min TTL of empty replays). Found in adversarial review.

Fix

• resetBackendGeneration() (extracted from ensureBackend's spawn branch) now also voids backendLoadedSessions.
• fetchMessagesForReplay settle-polls the first replay read of a backend generation (no hydrationWatermark entry — presence, not value, is the observed-settled marker); raiseWatermark materializes the entry at 0; create/fork seed 0 (a fresh store is complete by construction). The turn-path baseline opts out (guardFirstRead:false) — no-watermark is unreachable there once stamps are void, and the settle's 2×300ms gaps must not sit on the prompt path.
• The turn finally re-stamps only while the subscribe-proven instance (residentBackend) is still the live backend.

Residual, accepted: a read landing above the watermark while a false-stable hydration still grows can still ship a prefix — the client-side last-message-id reconciliation is that defense (separate repo).

Verification

• Full suite 2087/2087 (three consecutive runs), tsc, eslint, prettier clean.
• New regression tests: no-watermark alreadyLive load re-settles a mid-hydration ladder (resume-single-flight); respawn voids stamps (session-eviction); failed backend-lost recovery leaves no stamp — verified red on the pre-fix code (turn-state).