feat(js2wasm): add embedded Wasmtime module backend

[ttraenkler]

Summary

Add an opt-in engine_js2wasm backend that preserves the public rusty_v8 API while executing ahead-of-time js2wasm artifacts through embedded Wasmtime 47.0.3.

The latest slice:

• keeps the exact pinned deno_core 0.407.0 wrapper/module/application transaction in one context-owned Wasmtime instance
• validates source hashes and order for primordials, infra, timers, core, ext:core/mod.js, and the exact hello_world usage
• implements the value semantics reached by unchanged Rust deno_core: handles, objects/prototypes, templates/functions, weak handles, TryCatch, promises, synthetic/source modules, metadata, namespaces, external backing stores, and typed arrays
• binds the real Rust-owned Deno.core.ops.op_sum and op_print Function handles through nine strict scalar imports while preserving their original External(OpCtx*) callback data
• binds generic AOT replay to both the exact module graph and exact precompiled bytes with SHA-256, failing closed on a missing, stale, or mismatched sidecar
• releases ContextState before reentrant Wasmtime callbacks and rejects recursive Store access cleanly
• retains a Unix-only 237-symbol weak, fail-loud diagnostic ABI so every unimplemented executed path names itself and aborts

With companion compiler PR loopdive/js2#4766, unchanged pinned deno_core::hello_world exits 0 and prints exactly:

The sum of
1,2,3
is
6
Exception:
TypeError: serde_v8 error: invalid type; expected: array, got: Number

Verification

• production and runtime-compiler/diagnostic feature checks passed; JSC-only dependency isolation also passed
• 11/11 nonignored js2wasm semantic integration tests passed
• public exact Script/Module/usage test passed and verifies stable Function identities, exact External data pointers, ordered 6-print/2-sum callbacks, inner exception consumption, and a clean outer TryCatch
• graph-binding regressions reject missing, wrong-graph, and wrong-artifact bindings before deserialization
• exact raw artifact precompile/bootstrap passed in two isolated stores
• unchanged pinned Rust example exited 0 with the six lines above and no diagnostic weak-symbol abort

Scope

This remains a narrow synchronous POC. It does not claim general Rust/Wasm object identity, module live bindings, broad generated op tables, async promise/microtask semantics, dynamic imports, inspector support, or a complete portable Deno distribution.

Related: loopdive/js2#4766 and littledivy#74.