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/daemon_controller.h

@@ -108,6 +108,7 @@ public:
     void setZapOnNextStart(bool enabled);
     bool zapOnNextStart() const;
     void setSalvageOnNextStart(bool enabled);
+    void setReindexOnNextStart(bool enabled);   // -reindex: rebuild the block DB from raw blocks on next start
 
     static ShutdownDecision evaluateShutdownPolicy(bool hasDaemon,
                                                    bool externalDaemonDetected,