Adds:
- retry.go: classifyError() splits errors into Permanent vs Transient
(used to gate auto-retry); isContextErr() detects ctx cancellation
through wrapping (OpError, errors.Join).
- hints.go: hintFor(testID, err) returns short Russian explanation per
failure step, with dedicated branches for SOCKS5 sentinels, every
documented REP code (0x01..0x08), STUN sentinels, timeouts, and a
friendly-name fallback.
Coverage: retry.go 100%, hints.go 100%; package total 94.2%.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
