Implement HTTP-layer cache for transport-level caching

[nicmostert]

Overview

Implement the HTTP-layer cache for whurl. This layer intercepts GET/HEAD requests, uses HTTP caching headers (ETag, Last-Modified, Cache-Control) to serve fresh or revalidated responses, and minimizes redundant network requests.

Acceptance Criteria

• Intercept all GET/HEAD requests via a transport/middleware.
• Compute cache keys using method, URL, and Vary headers.
• Store responses with status, headers, body, ETag, Last-Modified, and computed TTL (Cache-Control/Expires).
• Prefer revalidation (If-None-Match/If-Modified-Since) over arbitrary TTL when possible.
• Respect Cache-Control directives (no-store, no-cache, private), Vary, and common cacheable statuses (200, 301, 404).
• Provide in-memory (LRU) backend, with optional disk persistence.
• Expose configuration: cache size, default TTL, backend selection.
• Implement explicit invalidation methods.
• Document integration patterns and supported options.
• Comprehensive tests for cache behavior, revalidation, and performance improvement.