feat(rust): Rust codegen backend (speckl#24)

- New generator: compiler/src/generators/rust.ts (652 lines)
- --target=rust CLI option produces full Rust crate (Cargo.toml + src/lib.rs)
- Types: Nat→u64, Bool→bool, String→String, Set→HashSet, Map→HashMap
- State variables → Rust struct fields
- Actions → Rust impl methods returning Result
- Expression rewriter: .contains(&x), .get(&k), .len(), etc.
- Tested against ToggleSwitch, TigerBeetleLedger, KafkaKRaft
- All 37 existing tests pass

WASM path: .speck → Rust → wasm-pack → WASM (now complete)

compiler/test-output/RetryHandler.prov.jsonld

@@ -12,7 +12,7 @@
         "@type": "prov:Generation",
         "prov:activity": {
           "@type": "prov:Activity",
-          "prov:startTime": "2026-05-10T06:07:50.697Z"
+          "prov:startTime": "2026-05-10T06:24:31.493Z"
         }
       },
       "rdfs:label": "RetryHandler",