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/clock/package.wit

@@ -0,0 +1,9 @@
+package wasmeld:clock@0.1.0;
+
+interface monotonic-clock {
+  now: func() -> u64;
+}
+
+world clock-host {
+  import monotonic-clock;
+}