feat(node): detect an unreadable block DB on startup and offer a one-click reindex

When a daemon update changes the block-index on-disk format (or the index is
corrupt), dragonxd aborts at startup — "non-canonical optional discriminant" →
"Error loading block database. Aborted." — and the wallet silently shows a zero
balance. Previously the connect loop just crash-restarted into the same abort up
to 3x and then reported a bare "Daemon crashed N times", with no path forward.

Now:
- daemon/daemon_startup_diagnosis.h: pure blockDbOutputLooksBroken() classifies
  the crashed node's captured console output (the fatal block-DB markers).
- The connect loop detects it on the FIRST abort, STOPS crash-restarting into the
  same failure (each retry reloads the whole index — wasteful), and offers a fix.
- A one-shot -reindex flag (EmbeddedDaemon::setReindexOnNextStart → DaemonController
  forwarder → args) rebuilds the block index + chainstate from the intact raw
  blocks; App::reindexBlockDatabase() arms it and un-gates the loop to restart.
- An auto-shown dialog (renderBlockDbReindexDialog) + a notification explain the
  situation ("your coins are safe; the node just can't load the chain") and offer
  a one-click "Rebuild block database". Full-node only (gated), lite-safe.

This is the exact trap behind a real "big wallet shows no funds" report: a
post-format-change daemon over pre-change chaindata. Reindex also fixes a plain
corrupt index.

Adds testBlockDbOutputDiagnosis (the abort sequence + individual markers trip it;
normal startup / wallet-corruption / asmap errors do not). Suite green (1/1).

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

src/daemon/embedded_daemon.h

@@ -206,6 +206,13 @@ public:
     void setSalvageOnNextStart(bool v) { salvage_on_next_start_ = v; }
     bool salvageOnNextStart() const { return salvage_on_next_start_.load(); }
 
+    // -reindex: rebuild the block index + chainstate from the raw blocks (blk*.dat) on startup. One-shot,
+    // consumed on the next start. Offered when the node aborts on an unreadable block database (a
+    // daemon-vs-chaindata format mismatch after an update, or a corrupt index). It implies a wallet
+    // rescan, so it's the block-DB analogue of -salvagewallet and coexists with the wallet-repair flags.
+    void setReindexOnNextStart(bool v) { reindex_on_next_start_ = v; }
+    bool reindexOnNextStart() const { return reindex_on_next_start_.load(); }
+
     /**
      * @brief One-shot isolated-datadir override for the NEXT start(): run the daemon against a
      *        different datadir (with its own DRAGONX.conf) plus the given extra args. Used by the
@@ -306,6 +313,7 @@ private:
     std::atomic<bool> rescan_on_next_start_{false};  // -rescan flag for next start
     std::atomic<bool> zap_on_next_start_{false};     // -zapwallettxes=2 flag for next start
     std::atomic<bool> salvage_on_next_start_{false}; // -salvagewallet flag for next start
+    std::atomic<bool> reindex_on_next_start_{false}; // -reindex flag for next start (rebuild block DB)
     std::string override_datadir_;                   // one-shot: -datadir for the next start
     std::vector<std::string> override_extra_args_;   // one-shot: extra args for the next start
     bool skip_port_check_ = false;                   // isolated instance on a non-default port