Slice A: shared-code seams for a cooperative single-threaded event backend

[scottmarchant]

Slice A of the WASI port series (see #3 for the combined change set and the slicing plan). Fork-internal draft: this is the upstream-facing shape for review here first; nothing points at upstream yet.

What this is

The two shared-code seams a cooperative single-threaded event backend needs, with no functional change on any current platform:

1. Poke-defer brackets. _dispatch_cooperative_pokes_defer()/_undefer() compile to ((void)0) unless a cooperative event backend defines the real versions. They bracket every critical section that can submit work while holding an internal lock: the dispatch_sync/dispatch_barrier_sync inline funnels, both dispatch_async_and_wait funnels (including the dispatch_block_create privdata entry that Swift's asyncAndWait(execute:) produces), _dispatch_barrier_trysync_or_async_f, dispatch_once initializers, object dispose, and the specifics-hash mutation in dispatch_queue_set_specific. On threaded platforms a poke wakes another worker, so the brackets change nothing. On a cooperative backend (arriving in slice B1) a poke may run the submitted work immediately on the submitting stack; without the brackets, that work runs beneath the caller's held lock, turning programs that are correct on every threaded platform into spurious deadlock crashes.

2. Main-queue drain hoist. _dispatch_main_queue_drain moves out of DISPATCH_COCOA_COMPAT into #if DISPATCH_COCOA_COMPAT || defined(__wasi__), with the two runloop/Darwin-only steps (runloop-handle once, thread-QoS override propagation) staying gated under DISPATCH_COCOA_COMPAT. This lets the cooperative backend drain the thread-bound main queue through the same code the CFRunLoop callback uses, instead of maintaining a 63-line copy kept "in sync" by a comment (which is what the combined branch had before this refactor). The compiled result on Darwin is unchanged; platforms with neither macro compile neither version, as before.

Why land this first

• It is the only slice touching code other platforms compile, and it is small enough to review in one sitting.
• We cannot fully validate the DISPATCH_COCOA_COMPAT restructure ourselves: the public macOS CMake build of this repository is broken upstream, so only Apple CI can compile that path. Landing this slice first forces that conversation while the stake is ~90 lines.
• Every later slice (B1: build + core runtime; B2: test suite; C: Swift overlay; D: event sources) depends on these seams or is independent of them; none of them touch shared code again.

Verification

• Linux container (clang/Ninja, aarch64): main and this branch build identically and pass the same 23/23 upstream tests.
• The full cooperative backend built on these exact seams passes 52/52 WASI tests on the combined branch (feat: WASI port V2 — combined branch: #2's runtime + #1's test scenarios + API-contract tests #3), including regression tests for each bracket site (sync-nested-async, async-and-wait, specific-destructor, dispose-path destructors) and for the shared drain.
• Darwin: the restructure is mechanically identity-preserving (the two gated steps sit under a macro that is always true there), but compiling it needs Apple CI - the ask that comes with this slice.

🤖 Generated with Claude Code

[scottmarchant]

Superseded by #5: the branch rename to prepare-cooperative-event-backend closed this PR (GitHub could not retarget the head ref). #5 carries the same change with the upstream-ready commit message and description.