ci: Build WebAssembly (wasm32-unknown-emscripten) wheels

[aborgna-q]

Warning

⚠️ This PR is an LLM-generated proof of concept ⚠️

Summary

Adds a wasm job to the Python wheels workflow that cross-compiles the tket Python extension to wasm32-unknown-emscripten, producing pyodide-compatible WebAssembly wheels. Implements #1415.

Targets Emscripten 5.0.3 / Python 3.14 (pyodide pyemscripten_2026_0 ABI), mirroring the proven hugr wasm-wheel pipeline.

Changes

• tket1-passes/build.rs: recognise the emscripten target (SupportedPlatform::Emscripten) and skip linking a system stdc++ — the em++ linker driver provides its own libc++. The wasm tket-c-api static archive is consumed via TKET_C_API_PATH.
• .github/workflows/python-wheels.yml: new wasm job — nightly Rust with -Zbuild-std, emsdk 5.0.3, Python 3.14 — that fetches the prebuilt emscripten tket-c-api from hugrverse-env via install-hugrenv-action with the new target: emscripten_wasm32 input (and packages: tket, since llvm has no wasm build).

Dependencies / sequencing

⚠️ This depends on Quantinuum/hugrverse-env#15 (adds the emscripten_wasm32 build + the target action input). Before the wasm job can succeed:

1. hugrverse-env#15 merges to main.
2. A hugrverse-env release is cut that includes the hugrenv-tket-emscripten_wasm32.tar.gz asset.
3. tket2's hugrenv.lock version points at that release.

Note: the wheels workflow only triggers on push to main, tags, or workflow_dispatch (not on PRs), so this PR won't build wheels on its own.

Open questions / known limitations

• pytket dependency: tket-py depends on pytket, which has no wasm wheel, so importing the wasm wheel in pyodide may require additional work (cf. jpacold's notes on the issue).
• Threading: rayon par_iter_mut paths won't work under single-threaded wasm; may need a feature-flag/fallback.
• The wasm job is intentionally not added to the release needs yet, pending a decision on whether to publish emscripten wheels to PyPI.

Draft — pending hugrverse-env#15.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.56%. Comparing base (0b7766d) to head (62a2c94).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1788      +/-   ##
==========================================
- Coverage   85.57%   85.56%   -0.02%     
==========================================
  Files         197      197              
  Lines       30964    30964              
  Branches    29528    29528              
==========================================
- Hits        26497    26493       -4     
- Misses       3192     3195       +3     
- Partials     1275     1276       +1     

Flag	Coverage Δ
rust	85.24% <ø> (-0.02%)	⬇️

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