ci: fix tests to use macOS and Windows runners

The previous workflow defined a matrix with goos/goarch targets but:
1. Never used those variables (they were completely ignored)
2. Ran all jobs on ubuntu-latest regardless of target OS
3. Tests only ran via goveralls, not explicitly

This meant darwin and windows tests were never actually executed -
all 14 matrix jobs ran identical Linux tests.

Changes:
- Split into three jobs: test, cross-compile, coverage
- test: Runs on actual macOS, Windows, and Linux runners
- cross-compile: Build/vet checks for arm, arm64, wasm (can't test natively)
- coverage: Separate job for goveralls on Linux
- Add explicit `go test -v -race` for visible test output

Co-Authored-By: Claude <noreply@anthropic.com>