feat(runtime): add resident Wasmtime actor runtime

- define versioned service and clock WIT contracts
- enforce import allowlists, fuel, epoch, memory, I/O, and mailbox limits
- keep one Store and Component Instance resident per serial Actor
- add echo, counter, fault, spin, and capability probe components
- cover lifecycle, concurrency, sandbox, and fault recovery behavior

wit/service/package.wit

@@ -0,0 +1,11 @@
+package wasmeld:service@0.1.0;
+
+world service-component {
+  variant service-error {
+    init-failed(string),
+    call-failed(string),
+  }
+
+  export init: func(config: list<u8>) -> result<_, service-error>;
+  export invoke: func(input: list<u8>) -> result<list<u8>, service-error>;
+}