HTTP caching (RFC 9111) — Cache-Control, ETags, conditional requests

[jaytaph]

• Parse response cache directives: Cache-Control, Expires, ETag, Last-Modified, Vary, Age
• Parse request cache directives: Cache-Control: no-cache, no-store, max-age, max-stale
• Send conditional requests: If-None-Match, If-Modified-Since
• Handle 304 Not Modified — serve from cache, update headers
• Respect Vary — cache separate entries per differing request header combinations