CI: run tests on Windows and macOS in addition to Linux

[EhabY]

Problem

We've hit multiple test failures that only surface on Windows (and potentially macOS):

• Path separator mismatches (/ vs \) in assertions
• Shell quoting differences (single quotes on Unix vs double quotes on Windows)
• execFile unable to spawn .cmd shims on Windows
• cross-env needed for environment variable syntax in npm scripts

These issues are caught late, only when someone manually runs tests on a non-Linux machine.

Proposal

Add Windows and macOS runners to the CI test matrix so cross-platform regressions are caught automatically.

References

• fix: Windows compatibility for tests, scripts, and Vitest Explorer #876 - recent fixes for all of the above