feat(cloud): bound the imported-replay prefix blind spot with a re-anchor cadence - #700

Problem

#692's bounded incremental path validates ordered turn ids, a Merkle frontier, and the reread overlap — but a historical rewrite that preserves every provider-native turn id outside the overlap is undetectable from the compact checkpoint, and the PR documented that blind spot as lasting "until another invariant fails or a future full re-anchor occurs". Nothing scheduled that re-anchor, so the window was unbounded.

Solution

ImportedReplayCheckpoint gains an optional incrementalPassCount (absent reads as 0, so persisted checkpoints stay compatible in both directions). Each bounded pass increments it; at IMPORTED_INCREMENTAL_REANCHOR_EVERY = 64 the preparer declines the checkpoint and the pass takes the existing full authoritative read:

• an intact prefix validates against the cursor's chain commitment, rides the ordinary delta append, and stamps a fresh count-0 checkpoint — no upload cost for the healthy case;
• a mutated prefix now fails the chain comparison at a bounded horizon (~64 appended turns) and pays the designed epoch rewrite.

The forced reread amortizes the O(total) read to under 2% of passes; it reuses the untouched full-read path rather than adding a new one.

Verification

• New engine test: the counter advances across bounded passes, an exhausted budget forces exactly one full read that still APPENDS (never rewrites an intact history), and the fresh checkpoint restarts at zero.
• Existing incremental/migration/shrink suites green (63 tests across the three files); pnpm typecheck clean.