fix(FN-4811): persist done-task integrity warnings across engine restarts

The periodic self-healing sweep at
SelfHealingManager.reconcileDoneTaskIntegrity() emits a single
'Integrity warning: done-task finalize evidence is unproven (<reason>)' log
entry per task when the task is in 'done' but has no provable on-main
evidence. Dedup was via an in-memory finalizeUnprovenWarned Set per manager
instance, so every engine restart resurfaced the same warning on the next
sweep — significant noise on done tasks that legitimately lack evidence,
typically residue of FN-4811 contamination (FN-4771/FN-4778 in production).

Adds an optional MergeDetails.integrityWarning = { warnedAt, reason } field
and persists it on the first warning. Both warning sites in
reconcileDoneTaskIntegrity() (the unproven-and-still-mergeable branch and
the unproven-final branch) now consult the persisted record:

  - Same reason as persisted → skip re-emitting, just rehydrate the in-memory
    Set for in-process consistency.
  - Different reason → re-warn (so a *new* classification problem still
    surfaces) and update the persisted record.

Tests added under
packages/engine/src/__tests__/reliability-interactions/integrity-warning-persisted-dedup.test.ts
(real-git, 4 cases):

  - First sweep: emits warning + persists record.
  - Second sweep, same instance: in-memory Set dedupes (existing contract).
  - Fresh manager (simulated engine restart) + pre-persisted record:
    persisted dedup suppresses re-emission.
  - Fresh manager + persisted record with different reason: must re-warn
    and overwrite the persisted reason.

Full engine suite: 308 files, 5041 tests pass, 1 skipped. Lint clean.

Fusion-Task-Id: FN-4811

.changeset/FN-4811-integrity-warning-persisted-dedup.md

@@ -0,0 +1,18 @@
+---
+"@runfusion/fusion": patch
+---
+
+fix(FN-4811): persist done-task integrity warnings across engine restarts
+
+`SelfHealingManager.reconcileDoneTaskIntegrity()` previously deduped its
+"Integrity warning: done-task finalize evidence is unproven" emissions via an
+in-memory `Set<string>` per manager instance. Every engine restart created a
+fresh manager, so the periodic sweep re-emitted the same warning for the same
+task on every cycle — producing significant log noise on done tasks legitimately
+lacking on-main evidence (often FN-4811 contamination residue).
+
+Adds an optional `integrityWarning: { warnedAt, reason }` field on
+`MergeDetails` and persists it on the first warning. Subsequent sweeps (within
+the same process or after restart) check the persisted reason and skip
+re-emitting an identical warning. A different classification reason still
+re-warns and updates the persisted record.