Add multi-platform coverage merging via Coveralls and Codacy

[Copilot]

Platform-specific modules (Windows, Darwin) showed 0% coverage in reports generated only from Linux CI. With tests running on 3 OS × 5 Python versions, coverage data exists but wasn't being aggregated.

Changes

Coverage Integration

• Upload coverage to Coveralls with parallel: true from all 15 matrix jobs, merge via coveralls-finish job
• Upload coverage to Codacy from all matrix jobs with per-job flag names ({os}-py{version})
• Generate both XML (Codecov/Codacy) and LCOV (Coveralls) formats in pytest

Dependencies & Configuration

• Add coveralls>=3.0 to dev dependencies
• Add coverage badges (Coveralls, Codacy) to README
• Exclude coverage.lcov in .gitignore

Result

Coverage reports now reflect actual multi-platform execution. Platform-specific code paths that only run on Windows/Darwin CI jobs are properly tracked and merged into overall coverage metrics.

Original prompt

Looking at the test coverage issue listed in CODE_REVIEW.md, one thing flagged is that the Windows specific paths is not tested on Linux - however, the pytest action does test the package on all platforms, so we do in fact have test coverage for Dawrin and Windows specific things. To allow accurate reporting of test coverage, can we add coveralls support to the pytest action and then ensure we somehow merge the coverage reports from all the platforms to get an accurate view of the actual test coverage.

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 1773c2f1	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (1773c2f)	Report Missing	Report Missing	Report Missing
Head commit (89170d6)	291	271	93.13%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#34)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩