Add Cache-Status, Content-Digest, and HSTS header parsing
Implement three new header parsing modules in header_parsing.ml:
Cache-Status (RFC 9211):
- Parse cache status entries with hit/fwd/stored/ttl parameters
- Support forward reasons: uri-miss, vary-miss, stale, bypass, etc.
- Helper functions for checking hits and getting forward reasons
Content-Digest / Repr-Digest (RFC 9530):
- Parse and generate digest headers with :base64: format
- Compute SHA-256 and SHA-512 digests using digestif
- Validate content against provided digests
- Get strongest available digest (prefer SHA-512)
Strict-Transport-Security (RFC 6797):
- Parse HSTS directives (max-age, includeSubDomains, preload)
- Case-insensitive directive matching
- Preset configurations for common use cases
- Helper to check if HSTS is effectively enabled
Add 20 new tests covering all three header types, bringing the
header parsing test suite to 44 tests total.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>