Unit tests + go test in CI (closes #12); README compatibility notes

[TheZeekA]

Closes #12.

Tests (src/unit_test.go, all pass locally; full suite ~50s)

• Reed-Solomon, for every codec used (rs1 through rs128): corrects every error count up to the limit, never silently returns the original past the limit, force-decode returns the right length, fast decode skips correction
• PKCS#7 pad/unpad round trip for all lengths, and corrupted final bytes left alone rather than crashing
• computeHeaderMAC: gives the same result every time, and changes when any header byte, the key or the HKDF salt changes
• Comment limit: 100,000 chars is rejected with no output left behind; 99,999 chars encrypts and reads back intact
• Malformed comment-length field (-0001, +1234, spaces, letters, NULs) is rejected with "Unable to read comments length" and no output

CI: pr-test-build-windows now runs go test . after the build, and also runs for PRs into testing, the branch CONTRIBUTING tells contributors to target.

Docs

• CONTRIBUTING: replaced "There's no test suite" with how to run it.
• README: added a Compatibility and upgrading section (v1.50/1.51 keyfile volumes don't open in v2.0.0; Picocrypt volumes aren't supported), mentioned the still-open PCC-004 finding under Security, and removed the three list items left dangling when the Acknowledgements section was deleted.