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.specbom.cdx.json

@@ -38,7 +38,7 @@
         "name": "Scott Scoble",
         "email": "scott@scoble.me"
       },
-      "timestamp": "2026-05-10T06:07:50.700Z"
+      "timestamp": "2026-05-10T06:24:31.495Z"
     },
     {
       "subject": "speck:RetryHandler",
@@ -53,7 +53,7 @@
         "name": "Speckl Compiler",
         "email": "compiler@speckl.scoble.me"
       },
-      "timestamp": "2026-05-10T06:07:50.700Z"
+      "timestamp": "2026-05-10T06:24:31.495Z"
     },
     {
       "subject": "speck:RetryHandler",
@@ -68,7 +68,7 @@
         "name": "Speckl Compiler",
         "email": "compiler@speckl.scoble.me"
       },
-      "timestamp": "2026-05-10T06:07:50.700Z"
+      "timestamp": "2026-05-10T06:24:31.495Z"
     }
   ]
 }