[Prototype] Batch consecutive await continuations on one fiber (follow-up to #7)

[jbaczuk-qualia]

Summary

Optional follow-up to #7 (stacked on it; merge #7 first). Adds one V8 entry point so node-fibers can run a chain of await continuations from fibered code on one fiber instead of switching stacks for each one. Pairs with qualialabs/node-fibers#8.

File	Change
deps/v8/include/v8-microtask-dispatch.h	RunNextDispatchableMicrotask(isolate) and the unmangled v8_qualia_RunNextDispatchableMicrotask
deps/v8/src/runtime/runtime-promise.cc	If the next job in the default microtask queue is a promise reaction job with CPED set, take it off the queue and run it on the current stack (through the same private-queue path as RunDispatchedMicrotask) and return true; otherwise return false and leave the queue alone

The embedder only calls it while the job #7 dispatched is still nested inside the microtask loop, and only takes jobs from the front, so the order of jobs is unchanged. The RunMicrotasks builtin re-reads the queue's start and size before every job, so the drain it's nested in carries on with the job after the ones taken. No builtin, runtime-function or snapshot changes beyond #7.

Results

Same arm64 dev-qualia container as #7, together with qualialabs/node-fibers#8 (FIBERS_AWAIT_REUSE=0 gives the #7 behaviour on the same binary):

	stock node 18	#7	#7 + this
chain of awaits in a fiber	0.05–0.07 us	~25–30 us	0.13 us
I/O-style await in a fiber (each resumes from its own macrotask)	~1.1 us	~33 us	~28 us
100k park/resume cycles	n/a	~109 us/cycle	~97 us/cycle
microtask drain per macrotask, code that never had a fiber	0.58 us	0.57 us	0.57 us
1e6 awaits outside any fiber	~40 ms	~40 ms	~43 ms

The I/O case is close to the floor for CORO_PTHREAD: a bare run() + yield() round trip is ~20 us in this Docker VM on stock node. In local qli5 qualia ~80% of dispatchable jobs since boot ran batched (46,624 dispatched, 227,679 batched); startup hooks that await and then block still work, and code that never had a fiber still doesn't get one.

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