fix: honor options.contextPointer in QuickJSAsyncRuntime.newContext

[xfy2412]

QuickJSAsyncRuntime.newContext silently drops options.contextPointer, while the base
QuickJSRuntime.newContext honors it:

// runtime.ts (base)
const ctx = new Lifetime(options.contextPointer || this.ffi.QTS_NewContext(this.rt.value, intrinsics), ...)

// runtime-asyncify.ts (before this PR)
const ctx = new Lifetime(this.ffi.QTS_NewContext(this.rt.value, intrinsics), ...)  // contextPointer ignored

Three call sites pass that option in order to wrap a context the engine already handed us —
executePendingJobs() and the module loader / normalizer callbacks in runtime.ts, all shaped like:

const context = this.contextMap.get(ptr) ?? this.newContext({ contextPointer: ptr })

On asyncify variants the override therefore creates a brand new JSContext instead of wrapping
the existing one:

• the caller gets the wrong realm (e.g. a module loader runs against a fresh, empty context), and
• nothing owns the new context, so it is never disposed. It survives until JS_FreeRuntime, which
  asserts list_empty(&rt->gc_obj_list) — an uncatchable WASM abort(), i.e. untrusted JS can
  kill the host process.

Relation to #269 and #248

While root-causing #269 (that same assertion, reachable from a plain promise job) we found that
executePendingJobs can reach this fallback with a garbage pointer: the out-parameter view is
created before QTS_ExecutePendingJob, and if the job grows the WASM memory the view is
detached, so typedArray[0] reads undefined rather than 0. The ctxPtr === 0 guard misses
undefined, contextMap.get(undefined) misses as well — and this override turns that miss into a
leaked context.

#248 fixes the detach half of that chain. This PR fixes the other half, which remains reachable
through the module-loader paths even with #248 applied.

For reference, the full chain reproduces deterministically (160k+ objects in a single promise job →
Aborted(Assertion failed: list_empty(&rt->gc_obj_list), at: quickjs.c, JS_FreeRuntime)); with both
halves fixed, 160k/200k/500k all tear down cleanly. I can share the reproducer if useful.

Tests

No test added: my environment can't run the suite (it needs a full pnpm install, which fails here
on an unrelated peer-dependency conflict). Happy to add one along the lines of "newContext({ contextPointer: ptr })
returns a context whose ctx.value === ptr on an asyncify runtime" — just tell me which file you'd
prefer it in.