Wasm module serialization: structured clone + dynamic worker loader support

[guybedford]

This implements serialization of WebAssembly.Module in two places: structured clone, and the dynamic worker loader modules bag — allowing compiled Wasm modules to be passed between workers without recompilation, per the WebAssembly Web API serialization steps.

The motivating flow is source phase imports into dynamic workers:

import source mod from './lib.wasm';

const worker = env.LOADER.load({
  compatibilityDate: "2026-08-13",
  mainModule: "index.js",
  modules: {
    "index.js": `
      import source mod from './lib.wasm';
      export default { /* ... WebAssembly.instantiate(mod) ... */ };
    `,
    "lib.wasm": mod,
  },
});

What was implemented:

• jsg::Serializer/Deserializer now implement the GetWasmModuleTransferId/GetWasmModuleFromId delegate hooks, carrying v8::CompiledWasmModules in Released::wasmModules. Support is opt-in (Options::supportWasmModules) since compiled code can only be shared in-process: structuredClone() enables it; RPC and storage serialization continue to throw DataCloneError at serialize time (matching the spec's forStorage behavior).
• The worker loader modules bag accepts a WebAssembly.Module as a module value, or { wasm: module } alongside the existing { wasm: bytes } form.
• WorkerSource::WasmModule carries an optional v8::CompiledWasmModule; both the legacy and new module registries use FromCompiledModule when present (the new registry seeds its existing compile cache), so the loaded worker shares compiled code with zero recompilation.
• jsg::V8Ref<v8::WasmModuleObject> is now unwrappable, with rtti and TypeScript type updates (Record<string, string | WebAssembly.Module | WorkerLoaderModule>).

Note that structuredClone(wasmModule) previously threw DataCloneError and now succeeds. This is intentionally not gated behind a compat flag as it is purely additive spec-conforming behavior, matching browsers (same-agent-cluster cloning).

Test coverage: end-to-end loader tests (source phase import → loader → import source + default import in the child, against both module registries, covering both accepted forms), and structured clone tests including instantiation of the clone and object identity deduplication.

In future, dynamic import() could work directly against these module objects as well, per the ESM Phase Imports proposal.