feat(wit): add versioned KV store package

Define wasmeld:kv/store@0.1.0 with get, set, and delete operations.

Expose explicit invalid-key, value-too-large, and operation-failed variants so components can handle bounded storage failures without relying on host-specific errors.

wit/kv/package.wit

@@ -0,0 +1,17 @@
+package wasmeld:kv@0.1.0;
+
+interface store {
+  variant kv-error {
+    invalid-key(string),
+    value-too-large(u64),
+    operation-failed(string),
+  }
+
+  get: func(key: string) -> result<option<list<u8>>, kv-error>;
+  set: func(key: string, value: list<u8>) -> result<_, kv-error>;
+  delete: func(key: string) -> result<_, kv-error>;
+}
+
+world kv-host {
+  import store;
+}