feat(compiler): Z3 SMT-LIB2 verification backend (speckl#21)

- New generator: compiler/src/generators/z3.ts (1,034 lines)
- Translates SpeckDL state → Z3 sort declarations
- Translates invariants → Z3 define-fun returning Bool
- Translates actions → Z3 transition predicates (enabled + step)
- BMC unrolling with --verify-depth=N (default: 10)
- CLI: --target=z3 or --target=all
- ToggleSwitch and TigerBeetleLedger generate valid SMT-LIB2
- KafkaKRaft spec generates valid SMT-LIB2 structure
- Complex forall/set expressions emit 'true' as placeholders (to be improved)
- All 41 existing tests still pass

This unblocks M1.2 (KRaft verification), M3.2 (Show HN), and M4.3 (OAuth verification).

compiler/test-output/RetryHandler.specbom.cdx.json

@@ -38,7 +38,7 @@
         "name": "Scott Scoble",
         "email": "scott@scoble.me"
       },
-      "timestamp": "2026-05-10T05:07:15.425Z"
+      "timestamp": "2026-05-10T05:28:00.667Z"
     },
     {
       "subject": "speck:RetryHandler",
@@ -53,7 +53,7 @@
         "name": "Speckl Compiler",
         "email": "compiler@speckl.scoble.me"
       },
-      "timestamp": "2026-05-10T05:07:15.425Z"
+      "timestamp": "2026-05-10T05:28:00.667Z"
     },
     {
       "subject": "speck:RetryHandler",
@@ -68,7 +68,7 @@
         "name": "Speckl Compiler",
         "email": "compiler@speckl.scoble.me"
       },
-      "timestamp": "2026-05-10T05:07:15.425Z"
+      "timestamp": "2026-05-10T05:28:00.667Z"
     }
   ]
 }