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.cpp

@@ -238,6 +238,7 @@ bool Settings::load(const std::string& path)
         loadScalar(j, "seed_migration_dest", seed_migration_dest_);
         loadScalar(j, "seed_migration_temp_dir", seed_migration_temp_dir_);
         loadScalar(j, "seed_migration_sweep_txid", seed_migration_sweep_txid_);
+        loadScalar(j, "seed_migration_sweep_opid", seed_migration_sweep_opid_);
         loadScalar(j, "auto_lock_timeout", auto_lock_timeout_);
         loadScalar(j, "unlock_duration", unlock_duration_);
         loadScalar(j, "pin_enabled", pin_enabled_);
@@ -505,6 +506,7 @@ bool Settings::save(const std::string& path)
     j["seed_migration_dest"] = seed_migration_dest_;
     j["seed_migration_temp_dir"] = seed_migration_temp_dir_;
     j["seed_migration_sweep_txid"] = seed_migration_sweep_txid_;
+    j["seed_migration_sweep_opid"] = seed_migration_sweep_opid_;
     j["auto_lock_timeout"] = auto_lock_timeout_;
     j["unlock_duration"] = unlock_duration_;
     j["pin_enabled"] = pin_enabled_;