feat: fork-frontier waves 1-3 — advanced V8 bindings

[yneves]

Summary

Implements the remaining fork-frontier techniques from doc 207, giving er and blindfox fine-grained control over V8's runtime behavior.

Wave 1 — Cooperative GC & microtasks

• AdjustExternalMemory — report Go-side allocations to V8's GC heuristic so it can schedule collections at the right time
• SetMicrotasksPolicy — control when V8 drains the microtask queue (Explicit / Scoped / Auto)
• EnqueueMicrotask — schedule JS functions as microtasks without the overhead of Promise.resolve().then()
• SetOOMErrorHandler — observe OOM events via a Go callback trampoline for diagnostics and graceful shutdown

Wave 2 — Zero-copy data & profiling

• ArrayBuffer — NewArrayBuffer (copy), NewArrayBufferAlloc (V8-sandbox-allocated), ArrayBufferGetBytes for single-copy Go↔JS binary data transfer
• ExternalStringResource — NewExternalOneByteString for zero-copy Latin-1/ASCII string injection into V8
• Named property interceptors — getter/setter callbacks on ObjectTemplate using an isolate callback registry, enabling virtual-property patterns
• HeapProfiler — TakeHeapSnapshot returning Chrome DevTools-compatible JSON for memory debugging

Wave 3 — ES module support

• CompileModule / Instantiate / Evaluate — full ESM lifecycle with Go-side resolve callback
• GetNamespace — access module exports after evaluation
• Import request introspection — GetModuleRequestsLength / GetModuleRequest to inspect import specifiers before instantiation
• ModuleStatus enum — complete lifecycle tracking

Deferred

• Fast API Callbacks (F4.1) — C++ CFunction templates are impractical to bridge through CGO
• Streaming Bundle Compile (F1.3) — requires background thread management for ScriptStreamingTask

Test plan

• 35 new tests across 9 test files covering all new APIs
• Full regression suite passes (go test ./... — 0 failures)
• V8 sandbox compatibility verified (ArrayBuffer uses V8-internal allocator)
• CGO trampoline patterns validated (OOM handler, interceptors, module resolver)
• Documentation updated: api-reference.md, architecture.md, maintaining.md, CHANGELOG.md