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/app.h

@@ -804,6 +804,10 @@ private:
     void pumpSeedMigration();      // main thread: pick up background progress/result each frame
     // Phase 2: sweep all legacy funds into the new wallet, then adopt it as the primary wallet.
     void refreshSeedMigrationBalance();  // query the legacy total (shown on the Sweep step)
+    // W3-3: the terminal callback for the sweep opid, shared by the initial submit and a resume
+    // re-track. `resumed` selects the failure behaviour: a fresh sweep that fails -> Error; a resumed
+    // opid the daemon no longer knows (stale) -> back to the dismissable Sweep gate (re-check balance).
+    std::function<void(bool, const std::string&)> makeSweepCompletionCallback(bool resumed);
     void beginSweepToSeedWallet();       // z_mergetoaddress ["ANY_TADDR","ANY_ZADDR"] -> dest
     void pollSweepStatus();              // Confirming step: poll sweep confirmations + legacy balance
     void beginAdoptSeedWallet();         // stop daemon -> swap wallet.dat -> restart with -rescan