feat(node): in-app "Rebuild wallet database" recovery for a BDB-inconsistent wallet

Automates the manual recovery that fixed a wallet.dat with stale Berkeley DB
extent metadata (the "main" subdb metapage records a low last_pgno while its live
data spans thousands of pages beyond it). A tolerant page-walk reads every record,
but the daemon's BDB verify rejects the file and auto-salvages it — finding nothing
and shrinking the wallet to empty on each restart (the salvage cascade that looks
like fund loss). Plain "Restore original" can't fix it (hands the same broken file
back → re-salvage); a rebuild must produce a fresh, consistent DB.

Pieces (Approach A from the design workflow — out-of-process helper keeps AGPL
Berkeley DB out of the GPLv3 GUI):
- util/wallet_file_probe.h: extractWalletBtreeRecords() — sibling to parseWalletBtree
  that collects raw (key,value) bytes (same bounds-checked, subdb-aware walk).
  Records copied verbatim → encrypted key material passes through as opaque
  ciphertext (no passphrase). Overflow-page values (only large tx history) are
  skipped + counted; a rescan rebuilds history — funds unaffected.
- tools/wallet_rebuild/main.cpp: dragonx-wallet-rebuild CLI — reads via the tolerant
  reader, writes the records into a fresh BDB "main" btree via libdb (DB_EXCL, never
  overwrites), prints a JSON summary. New BDB-guarded CMake target.
- App::rebuildWalletDatabase(): picks the largest readable wallet/.bak as source,
  stops the daemon, runs the helper, VERIFIES the output (readable BDB with keys)
  before swapping, moves the current wallet aside (kept, timestamped), installs the
  rebuilt one, clears the stale BDB env, sets -rescan, restarts. Copy/rename only —
  never deletes. Result surfaced via the existing pumpWalletRestore channel.
- Wired as the preferred action on the existing wallet-auto-recovery dialog
  (shown only when the helper is present). Full-node only; lite-safe.

Verified end-to-end against the real broken wallet: helper reads 3,808 t-keys + 1
z-key + HD seed and the daemon LOADS the rebuilt output with no salvage. Adds
extractWalletBtreeRecords coverage. Build clean, suite green (1/1).

Remaining (follow-up): release packaging — build.sh bundling the helper built
against the vendored per-platform static libdb (DRAGONX_BDB_ROOT), and a macOS
Berkeley DB port (no in-tree artifact).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.h

@@ -1345,7 +1345,9 @@ private:
     void reindexBlockDatabase();          // restart the daemon with -reindex to rebuild the block DB
     void renderWalletRecoveredDialog();   // warn that the node auto-recovered/salvaged wallet.dat
     void restoreOriginalWallet();         // swap the wallet.<ts>.bak back over the salvaged copy + restart
-    void pumpWalletRestore();             // main-thread: surface the restore op's result
+    void pumpWalletRestore();             // main-thread: surface the restore/rebuild op's result
+    void rebuildWalletDatabase();         // rebuild a BDB-inconsistent wallet into a loadable one (helper)
+    bool walletRebuildAvailable() const;  // the dragonx-wallet-rebuild helper is present
     void processDeferredEncryption();
 
     // Private methods - connection