Add macOS coverage to Go CI

[Copilot]

Description

CI only exercised the Go module on Linux, so darwin-only build tag regressions could merge unnoticed. This adds PR-time macOS coverage for the Go module.

• Go CI matrix
  • Adds go-build-test with ubuntu-24.04 and macos-latest.
  • Runs both compile and test coverage:

os: [ubuntu-24.04, macos-latest]
run: go test ./...
run: go build ./...

• Regression coverage
  • Adds a focused workflow test that asserts the macOS matrix and Go commands stay in CI.

Type of change

• Bug fix
• New feature
• Documentation
• Maintenance / refactor

Testing

• Tested locally
• All existing tests pass
• Added tests for new functionality (if applicable)

[mrjf]

@copilot fix ci

[Copilot]

@copilot fix ci

Fixed in 256297c. The macOS Go CI failure was the reflink package excluding darwin from its fallback implementation; darwin now gets the generic fallback and the package cross-compiles for macOS.