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.prov.jsonld

@@ -12,7 +12,7 @@
         "@type": "prov:Generation",
         "prov:activity": {
           "@type": "prov:Activity",
-          "prov:startTime": "2026-05-10T05:07:15.423Z"
+          "prov:startTime": "2026-05-10T05:28:00.665Z"
         }
       },
       "rdfs:label": "RetryHandler",