Clarify why the Windows MSYS2 job excludes jsonschema

[Copilot]

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Description

The Windows GitHub Actions job was failing during dependency installation after jsonschema was added to the MSYS2-specific package subset. On that runner, jsonschema pulls in rpds-py, whose maturin build path rejects the x86_64-cygwin platform.

• Windows workflow constraint

  • Document the existing Windows/MSYS2 dependency subset as intentionally excluding jsonschema.
  • Call out the specific failure mode (rpds-py / maturin on x86_64-cygwin) inline in build.yml.

• Future-proofing

  • Make the reason for the reduced Windows dependency set obvious at the install site so the failing package is not reintroduced casually.

# Pick the subset we need for these builds, but avoid jsonschema here:
# its rpds-py dependency builds with maturin, which rejects x86_64-cygwin.
pip install --break-system-packages cascadetoml jinja2 typer click intelhex

[dhalbert]

This fix is useless; I had to add jsonschema, without it the build fails.

[dhalbert]

@copilot https://packages.msys2.org/packages/mingw-w64-x86_64-python-maturin exists, so I don't understand your diagnosis