Rename all 594 test data files from .json to .hujson and add
descriptive header comments to each file documenting what policy
rules are under test and what outcome is expected.
Update test loaders in all 5 _test.go files to parse HuJSON via
hujson.Parse/Standardize/Pack before json.Unmarshal.
Add cross-dependency warning to via_compat_test.go documenting
that GRANT-V29/V30/V31/V36 are shared with TestGrantsCompat.
Add .gitignore exemption for testdata HuJSON files.
Add golden test data for via exit route steering and fix via exit grant compilation to match Tailscale SaaS behavior. Includes MapResponse golden tests for via grant route steering verification.
Updates #2180
