Type-check with ty, pyrefly, pyright, mypy and zuban

[zsimic]

• 5 type checkers now run in CI (ty, pyrefly, pyright, mypy, zuban), all at zero findings
• Fixed what they found, by tightening the design rather than silencing them (no # type: ignore)
• Bug fixes found along the way: cpython-use-github: true crashing when the GitHub API call fails, and a missing bin/python crashing instead of being reported clearly
• Library use: BuildSetup() now takes a PythonSpec, see required_python_spec()
• Bumped GitHub Actions, changelog moved to a top-level CHANGELOG.md
• Details in docs/typecheck-v2.1.0.md

🤖 Generated with Claude Code

[coveralls]

Coverage Report for CI Build 38089273063

Coverage increased (+1.5%) to 99.893%

Details

• Coverage increased (+1.5%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 3 coverage regressions across 2 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

3 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
portable_python/init.py	2	99.64%
test_inspector.py	1	99.36%

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Coverage Stats

Relevant Lines:	2810
Covered Lines:	2807
Line Coverage:	99.89%
Coverage Strength:	4.99 hits per line

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💛 - Coveralls