Varnish VCL parity checklist

[roboshyim]

Tracking parity with https://raw.githubusercontent.com/shopware/varnish-shopware/refs/heads/main/rootfs/etc/varnish/default.vcl\n\n- [ ] Cookie parsing: set sw-cache-hash from sw-cache-hash cookie when header missing\n- [ ] /widgets/checkout/info 204: match VCL logic exactly\n- [ ] BAN semantics: implement ban list and evaluate on lookup\n- [ ] PURGE-by-URL: implement url->keys index (respect context variants)\n- [ ] Hit-for-miss / uncacheable caching for sw-dynamic-cache-bypass\n- [ ] Grace/stale revalidation strategy (serve-stale + background refresh)\n- [ ] Streaming proxying (avoid buffering entire bodies)\n- [ ] ESI support plan (optional)\n- [ ] Observability: metrics + tracing fields\n\nNotes: docs/varnish-parity.md

[roboshyim]

Noted deployment constraints from Shyim: single instance, start with disk cache, TLS/HTTP2 terminated upstream.\n\nImplemented initial disk-backed cache using sled index + per-entry files (cache dir env: CODYCACHE_CACHE_DIR).\n\nNext: tighten VCL parity (cookie->sw-cache-hash extraction, /widgets/checkout/info exact logic, BAN list, URL PURGE index).

[roboshyim]

Updates:

• Cookie parsing: set sw-cache-hash from cookie when header missing (merged PR Derive sw-cache-hash from cookie #8)
• /widgets/checkout/info 204: implemented (and improved via request normalization) (PR Derive sw-cache-hash from cookie #8)
• BAN semantics: minimal ban list persisted in sled + evaluated on lookup (PR Implement BAN (pattern-based invalidation) #10)
• PURGE-by-URL: url->keys index (normalized URL) + PURGE invalidates variants (PR Implement PURGE by URL #9)
• Hit-for-miss / uncacheable caching for sw-dynamic-cache-bypass (still TODO)
• Grace/stale strategy: stale-if-error (PR Serve stale (grace) only on upstream failure #11). (Background refresh not implemented.)
• Streaming proxying (still buffers full bodies)
• ESI support plan (optional)
• Observability: metrics + tracing fields

Next suggested focus: implement hit-for-miss handling for sw-dynamic-cache-bypass and add minimal observability headers (X-Cache HIT/MISS/STALE/BYPASS).

[roboshyim]

Merged PR #12: implements hit-for-miss for sw-dynamic-cache-bypass and adds x-codycache observability header.

[roboshyim]

Merged PR #13: streaming proxying for non-cacheable/bypass responses (reduces buffering).