[skwasm] Implement Wasm worker futex semaphores - #194139

Replaces Emscripten's no-op `-sWASM_WORKERS=1` POSIX semaphore stubs
(`sem_init`, `sem_destroy`, `sem_post`, `sem_wait`, `sem_trywait`) with
a WebAssembly atomic futex implementation in multi-threaded Skwasm and
Web Impeller (`wimp`) builds.

### Rationale

Under Emscripten's `-sWASM_WORKERS=1` runtime without `-pthread`
(`skwasm_heavy` / `wimp_heavy`), `libc-ww.a` compiles
`system/lib/pthread/library_pthread_stub.c`, which stubs `sem_init`,
`sem_wait`, `sem_trywait`, `sem_post`, and `sem_destroy` (as well as
`emscripten_futex_wait` / `emscripten_futex_wake`) to `return 0`.

In Skia (`third_party/skia/src/core/SkSemaphore.cpp`), `SkSemaphore`
uses POSIX `sem_t` on `__EMSCRIPTEN__` as the contended slow-path
backing for `SkMutex` and `SkSharedMutex`. Because `sem_wait` is a
no-op, any contended acquisition on FreeType's global `f_t_mutex()`
(`third_party/skia/src/ports/SkFontHost_FreeType.cpp:221-226`) or
`SkTypeface_FreeType::fC2GCacheMutex`
(`third_party/skia/src/ports/SkTypeface_FreeType.h:87`) returns
immediately when the browser main UI thread performs `Paragraph::layout`
/ `MakeTextFrameFromTextBlobSkia` concurrently with the background
render worker rasterizing glyphs from the shared `FT_Library` /
`FT_Face`, causing memory corruption and worker stalls
([flutter#193695](flutter#193695)).

### Summary of Changes

-
[`engine/src/flutter/skwasm/wasm_worker_semaphore.cc`](https://github.com/kevmoo/flutter/blob/87e66a27b6b/engine/src/flutter/skwasm/wasm_worker_semaphore.cc)
&
[`engine/src/flutter/skwasm/BUILD.gn`](https://github.com/kevmoo/flutter/blob/87e66a27b6b/engine/src/flutter/skwasm/BUILD.gn):
Override `sem_init`, `sem_destroy`, `sem_post`, `sem_wait`, and
`sem_trywait` via linker `-Wl,--wrap=sem_*` in multi-threaded
`skwasm_variant` (`#if defined(__EMSCRIPTEN_WASM_WORKERS__)`) builds:
- Stores the atomic counter inline in `sem_t`'s 16-byte `__val` buffer
(`cache/sysroot/include/semaphore.h:17-19`), requiring zero heap
allocation or teardown in `sem_destroy`.
- Background `DedicatedWorker` threads
(`emscripten_wasm_worker_self_id() != 0`, cached in a `thread_local
bool` to avoid a Wasm-to-JS FFI call per contended wait) sleep via
`emscripten_atomic_wait_u32` and wake via `emscripten_atomic_notify`.
- The browser main thread (`emscripten_wasm_worker_self_id() == 0`,
where `memory.atomic.wait32` / `Atomics.wait` is disallowed by the
WebAssembly Threads spec because `[[CanBlock]] == false`) uses an atomic
compare-exchange spin-acquire loop.

### Verification

- Built `wasm_release:skwasm/skwasm_heavy.js` and
`wasm_release:skwasm/wimp_heavy.js` and verified `20/20` multi-threaded
text-grid runs (`grid@100` and `grid@1000`) complete without `FT_Face`
memory corruption or worker stalls in Chrome 154.

Fixes flutter#193695

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…188628)

Related: flutter#187660. Part 1 of 5 from the original hybrid-rasterizer
change. This PR is now limited to the independent benchmark
instrumentation.

Adds `presentation_duration` around preparation and frame submission,
including asynchronous raster work, and records it in the web benchmark
harness. The timer stops before reporting, and benchmark listeners are
cleaned up even when startup fails. The existing profile JSON schema is
unchanged.

Verified locally: two engine timing tests cover enabled/disabled
profiling and delayed preparation/rasterization; a browser regression
test covers listener cleanup after synchronous startup failures. Static
analysis and formatting pass. This change is independent of the hybrid
rasterizer.

The remaining pieces are separate drafts:

- flutter#195103: native callback IDs and optional bitmap capture.
- flutter#195104: platform-view DOM commit after rasterization.
- flutter#195105: opt-in hybrid presentation, depending on the two changes
above.
- flutter#195106: desktop default policy and benchmark results, depending on
the opt-in implementation.

The duplicate semaphore and codec changes were dropped on rebase because
master already includes flutter#194139 and flutter#193187.

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responsibilities, or I am not using AI tools.
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responsibilities.
- [ ] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant in-code documentation (doc comments with
`///`).
- [ ] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

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Adds a manually dispatched "Web deploy" workflow: scripts/build_web.sh on
ubuntu with the Flutter release given as an input (default 3.51.0-0.1.pre on
beta, which carries the multi-threaded Skwasm futex fix, flutter/flutter#194139,
for the text-layout/glyph-raster race 3.50.0 still has, #193695), then
scripts/deploy_web.sh from main in the release environment. The deploy is
tagged with the app version and messaged "summary; sha; version; webapp
only", like the hand-run deploys, and checked against the active Worker
version. Building alone works from any branch.

The deploy job needs CLOUDFLARE_API_TOKEN and CLOUDFLARE_ACCOUNT_ID in the
release environment.

Co-Authored-By: Claude Opus 5.5 <[email protected]>