feat(z3): zero-error SMT-LIB2 output + BMC verify blocks for KRaft

- Declare speckl_len_Int helper for list/sequence lengths
- Fix infix = inside => bodies (final pass regex)
- Handle prime notation: currentTerm' → currentTerm_post
- Handle length() calls: length(log) → (speckl_len_Int log)
- Handle min(): (ite (<= a b) a b) instead of unsupported 'min'
- Handle subtraction: i-1 → (- i 1)
- Declare _post state vars when invariants compare pre/post
- Added 3 BMC verify blocks to KRaft spec (TermMonotonicity, CommitSafety, LeaderCompleteness)
- Z3 returns clean sat with zero errors for all specs
- All 37 tests pass

examples/KafkaKRaft.speck

@@ -145,4 +145,18 @@ speck KafkaKRaft {
         | BecomeLeader
         | AppendEntries
         | HandleAppendEntriesRequest
+
+    // --- Bounded Model Checking ---
+
+    verify Always(TermMonotonicity) {
+        depth 10
+    }
+
+    verify Always(CommitSafety) {
+        depth 10
+    }
+
+    verify Always(LeaderCompleteness) {
+        depth 10
+    }
 }