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",

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"
     }
   ]
 }

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

@@ -13,7 +13,7 @@
     "spdxVersion": "3.0.1",
     "dataLicense": "CC0-1.0",
     "creationInfo": {
-      "created": "2026-05-10T06:07:50.702Z",
+      "created": "2026-05-10T06:24:31.496Z",
       "createdBy": [
         "Organization: Speckl",
         "tool: speckl-compile-0.2.0"
@@ -27,7 +27,7 @@
           "@type": "spdx:Tool",
           "name": "speckl-compile"
         },
-        "timestamp": "2026-05-10T06:07:50.702Z",
+        "timestamp": "2026-05-10T06:24:31.496Z",
         "comment": "Review type: manual"
       }
     ]