[Prototype] Node 24: let node-fibers run await continuations on a fiber (port of #7 + #9)

[jbaczuk-qualia]

Summary

Port of the node 18 fiber-await prototype (#7 + #9) to node 24.21.0 / V8 13.6, stacked on #6 (custom-v24-pthread; merge #6 first). Lets node-fibers run the code after an await (and .then callbacks) registered inside a fiber on a hidden fiber, and batch runs of them on one fiber. Pairs with qualialabs/node-fibers#9. One commit, V8 only:

File	Change
deps/v8/include/v8-microtask-dispatch.h (new, identical to #7/#9)	SetMicrotaskDispatchCallback, RunDispatchedMicrotask, RunNextDispatchableMicrotask and unmangled v8_qualia_* entry points for dlsym
deps/v8/src/builtins/builtins-microtask-queue-gen.cc	Before a fulfill/reject reaction job whose continuation-preserved embedder data (CPED) is set, call Runtime::kDispatchMicrotask; skip the job if the embedder took it
deps/v8/src/runtime/runtime-promise.cc, runtime.h	Same logic as node 18: private one-job queue run through the RunMicrotasks builtin, plus batching of the dispatchable jobs queued right behind

What differs from node 18

• API renames (Tagged<>, DirectHandle, RETURN_FAILURE_IF_EXCEPTION, new TryRunMicrotasks signature): mechanical.
• CPED lives on the isolate. V8 13's MicrotaskQueue::RunMicrotasks already saves and restores the caller's CPED around a drain, so unlike [Prototype] Node 18: let node-fibers run await continuations on a fiber (V8 microtask dispatch) #7 the builtin needs no CPED change; the private one-job runner does the same save/clear/restore.
• AsyncContextFrame shares the slot. Node 24 keeps AsyncLocalStorage state in CPED unless started with --no-async-context-frame. node-fibers only enables dispatch with the flag (fibers needs it on node 24 anyway); the tag it uses is true, which lives in read-only space, so the per-archived-thread GC visit of the CPED root (the unapplied 4172f779 fix) can't corrupt it.
• Forward risk: if a future node drops --no-async-context-frame, tagging through CPED stops being possible and this would need its own field on promise reactions (a larger V8 patch).

Testing

Built incrementally on the custom-v24-pthread tree (arm64, Docker), with qualialabs/node-fibers#9, all under --no-async-context-frame:

• Behaviour checks (fiber-await/test.js on node-fibers jbaczuk/fiber-await-v24-stats): 11/11 with reuse on and off; dispatch off fails the expected 7, same as Node 24.21.0 for CORO_PTHREAD fibers (custom-v24-pthread): three node patches, no V8 changes #6's stock build.
• node-fibers test/: all pass. znewsham's scenarios from node-fibers#7 (with the running fiber): 9/9, including the 100k park/resume test. meteor-promise suite with shared-packages#265: 15/15 (both promise implementations).
• GC stress: behaviour checks pass under --stress-scavenge=20 and --stress-compaction.
• fiber-await/bench.js: fiberless awaits 0.03–0.04 us (stock Node 24.21.0 for CORO_PTHREAD fibers (custom-v24-pthread): three node patches, no V8 changes #6: 0.03), chained awaits in a fiber 0.10 us, I/O-style awaits ~28 us (the pthread run()+yield() floor is ~22 us), drains unchanged.
• GDC on qli5 locally (qualia branch jbaczuk/node-24-gdc-fiber-await, on top of qualia#56473): boots; a startup hook that awaits and then makes a sync Mongo call works on a hidden fiber; fiberless code still throws Can't wait without a fiber.

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