[Prototype] Run await continuations from fibers in hidden fibers (needs node#7)

[jbaczuk-qualia]

Summary

R&D prototype, pairs with qualialabs/node#7 (node 18 V8 patch). Makes the code after an await (and .then callbacks) that was registered inside a fiber run on a fiber, so fiber-only calls work there instead of throwing Can't wait without a fiber. One commit on async-resource:

File	Change
src/fibers.cc	SwapContext() sets the context's continuation-preserved embedder data (CPED) to a marker while a fiber runs, so V8 tags every promise reaction registered inside it. DispatchMicrotask() hands tagged jobs to a JS dispatcher; runDispatched(token) runs one. New Fiber.__setMicrotaskDispatcher and Fiber.microtasksDispatched. The V8 API is resolved with dlsym, so this binary still loads (feature off) on stock node
fibers_async.js	Dispatcher: runs each tagged job in a new hidden fiber. Fiber.current reports hidden fibers as undefined; new Fiber.currentIncludingHidden returns them. FIBERS_AWAIT_DISPATCH=0 disables dispatch
future.js	Future.wait uses Fiber.currentIncludingHidden so it can block in a hidden fiber

Why hidden fibers

A first version used ordinary fibers and broke qualia's boot: Fiber.current is also used as an API mode switch ("in a fiber, return the result synchronously, otherwise return a Promise"), e.g. @znewsham/meteor-mongo-collection-hooks cursor.observeChanges. Async code after an await suddenly got a handle instead of a Promise (platform-data-subscriptions/server.js:96: handlePromise.catch is not a function). Hiding the fiber keeps every Fiber.current check answering exactly as it does today; only blocking primitives that opt in (Future.wait, Promise.await via qualialabs/shared-packages#265) see the fiber.

Testing

On the node 18 build from qualialabs/node#7, arm64 dev-qualia container:

• Standalone checks: 11/11 pass (await then fiber-only call, nested awaits, .then, AsyncLocalStorage across the hop, Promise.all overlap, errors, Fiber.current still undefined after await, a dual-mode API still returns a Promise after await, fiberless code untouched, microtask order). FIBERS_AWAIT_DISPATCH=0 fails the expected 7.
• On stock node 18 the new binary loads with __setMicrotaskDispatcher absent and behaves as before.
• test/: identical results on stock vs patched node (pool.js/cleanup.js segfault on both: the pthread coro_destroy bug fixed in d3260f6 on node24-pthread).
• Local qli5 qualia boots; a startup hook doing await OrdersAsync.countDocuments() then Orders.find().count() logs both counts.

Known gaps

• ~25 @meteor guards that throw without a fiber (accounts, sync Meteor.call) still check Fiber.current, so they throw after an await as they do today.
• Legacy sync-style code that calls a dual-mode API after an await still gets a Promise (unchanged from today).
• Cost: every resumed reaction registered in a fiber hops onto a fiber, including internal ones such as the resolve callback Promise.await registers. meteor-promise's fiber-count test (should not double-wrap callbacks, native Promise) sees one extra fiber on the patched node (passes with dispatch off). Not measured yet.
• No version bump; nothing published.

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