feat(migrate): persist the sweep opid so a mid-sweep interruption can resume (W3-3)

Migrate-to-seed submits z_mergetoaddress -> an async opid, then only persists the resolved
txid once the op completes. An app-close during Sweeping (opid submitted, txid not yet
resolved) dropped the opid and resumed at the re-sweep gate, silently losing the tx.

Now the opid is persisted and re-tracked on resume. If the daemon forgot it (restart —
its op queue is in-memory only), the existing poller flags it stale and the callback falls
back to the dismissable Sweep gate; it can never hang (a thrown RPC aborts the poll, so a
stale classification only comes from a *successful* poll that omits the opid).

- New seed_migration_sweep_opid setting; adopted atomically with clearing any prior txid in
  the SAME settings.save(), and only once the submit succeeds — so a failed "Sweep remaining"
  re-sweep keeps the already-mined first sweep's Confirming context, and txid/opid are never
  both authoritative (resume checks txid first; torn-write safe).
- Resume routing extracted to a pure, unit-tested helper
  (data/seed_migration_resume.h::decideSeedMigrationResume): txid -> Confirming; opid AND
  connected -> re-track (Sweeping); else -> the dismissable Sweep gate. The connectivity gate
  keeps a disconnected resume out of the buttonless Sweeping spinner.
- Shared makeSweepCompletionCallback(resumed): success -> Confirming; resumed-stale -> Sweep
  gate (re-fetch balance + "may have already completed" copy); fresh-fail -> Error.

Fund safety unchanged: adopt still gated on legacy balance ~0 AND sweep tx mined; legacy
wallet.dat only ever moved to a never-deleted timestamped .bak.

Reviewed in two adversarial rounds (design + implementation) per the migration-code mandate;
both safety facts (no fund loss, no hang) held, and the resume-UX traps they surfaced are
fixed. Build-clean; ctest 1/1 (adds testSeedMigrationResume). See docs/wallet-hardening.md.

*** Still requires a live mainnet interrupted-sweep run before release (human gate). ***

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config/settings.h

@@ -356,6 +356,11 @@ public:
     // migration is past the sweep, so a resume goes to the confirm/adopt stage (not sweep again).
     std::string getSeedMigrationSweepTxid() const { return seed_migration_sweep_txid_; }
     void setSeedMigrationSweepTxid(const std::string& v) { seed_migration_sweep_txid_ = v; }
+    // W3-3: the async sweep operation id, persisted while the sweep is in flight (before it resolves
+    // to a txid). Lets a resume re-poll a mid-sweep interruption instead of dropping the txid. Cleared
+    // in the same write that persists the txid, so the txid always outranks it (see [[decideSeedMigrationResume]]).
+    std::string getSeedMigrationSweepOpid() const { return seed_migration_sweep_opid_; }
+    void setSeedMigrationSweepOpid(const std::string& v) { seed_migration_sweep_opid_ = v; }
 
     // Security — auto-lock timeout (seconds; 0 = disabled)
     int getAutoLockTimeout() const { return auto_lock_timeout_; }
@@ -587,6 +592,7 @@ private:
     std::string seed_migration_dest_;
     std::string seed_migration_temp_dir_;
     std::string seed_migration_sweep_txid_;
+    std::string seed_migration_sweep_opid_;
     int auto_lock_timeout_ = 900;   // 15 minutes
     int unlock_duration_ = 600;     // 10 minutes
     bool pin_enabled_ = false;