[b-re-w]

Refs #175 (step 1: the build experiment). Owner approval 2026-11-13: "#175 may add the new wasm 3.15t CI build job".

What

• .github/workflows/build-ft-wasm.yml runs on dispatch, on push to develop that touches its own paths, and on pull requests that touch them. Its dispatch inputs are python_version, emscripten_version (empty means CPython's own pin, 4.0.19 for 3.15.0) and trampoline_patch (apply or skip). It uploads one artifact and publishes no release.
• .github/scripts/ft-wasm/build-wasm.sh is separate from python-multiplatform/scripts/wasm/build-cpython.sh. That script is pinned to 3.14.2 and Emscripten 5.0.3, and feeds the wasmJs test runtime, which this job must not touch. The build uses CPython's Platforms/emscripten driver with --disable-gil --enable-wasm-pthreads, -pthread -sPTHREAD_POOL_SIZE=8 -sMAXIMUM_MEMORY=2GB, libffi and mpdec built with -pthread, and a small asserted patch so the call trampoline falls back to CPython's JS trampoline when memory is shared.
• Checks that fail the job:
  • Py_GIL_DISABLED in the build Python and in the host pyconfig.h;
  • atomics in libffi and mpdec;
  • shared memory in python.wasm;
  • stdlib name python3.15t;
  • smoke.py under node (GIL disabled, 4 threads join with correct results), in the build tree and again from the unpacked archive.
• docs/platforms/free-threaded-wasm.md has a "Not verified" list. wasm-design.md gets a pointer to it.

The Emscripten pin, the flags and the patch are experiment choices (agent proposal, not confirmed). The pin is still open: CPython 3.15.0 pins 4.0.19, while our wasmJs runtime uses 5.0.3.

Expected

This has never run anywhere. The likely first hard failure is the make-host link. This PR's own workflow run is the experiment.

Not in this PR (later #175 steps)

• Kotlin/Wasm's shared memory import.
• The worker pool for the first Thread.start from Kotlin.
• Function tables on every Worker for upcalls.
• Staging this archive as the wasmJs runtime.
• Cross-origin isolation headers and service worker.

Category: feature added (CI build experiment), documentation corrected.