fix(engine): log self-built block verification failures instead of dropping them silently

A block the engine builds itself that fails its own Verify() was dropped with a bare
continue — no log at any level. Because the VM re-notifies while its txs remain in the
mempool, the engine rebuilds and re-drops indefinitely: an invisible hot loop (observed
4.07M consecutive 'built block' lines with the chain stuck at height 1 and zero errors
logged). Dropping an unverifiable block is correct; doing it silently is not.

engine/chain/engine.go

@@ -3798,6 +3798,19 @@ func (t *Transitive) buildBlocksLocked(ctx context.Context) error {
 		// is never added to consensus, so IsAccepted cannot return true for it.
 		t.mu.Unlock()
 		if err := vmBlock.Verify(ctx); err != nil {
+			// A block we built ourselves that fails its OWN verification is a real fault
+			// (VM state, proposer/epoch rule, timestamp window...). Dropping it silently left
+			// the node in an INVISIBLE HOT LOOP: the VM keeps re-notifying while its txs sit in
+			// the mempool, so the engine rebuilds → drops → rebuilds forever — no block ever
+			// enters consensus and NOT ONE log line says why (observed: 4.07M consecutive
+			// "built block" lines, chain stuck at height 1, zero errors logged). The drop itself
+			// is correct (an unverifiable block must never reach consensus); the SILENCE is the
+			// bug. Log the fault so it is diagnosable at the point of failure.
+			t.log.Error("built block failed verification — dropping",
+				"blkID", vmBlock.ID(),
+				"height", vmBlock.Height(),
+				"parentID", vmBlock.ParentID(),
+				"error", err)
 			t.mu.Lock()
 			continue
 		}