encode: add standard benchmark and conformance harness (#7804)

This is a shared encode_test harness with HTML/JSON/JS/CSS payloads taken from caddyserver.com

Benchmarks:
- BenchmarkStandardEncodingPayloads: raw encoder NewEncoder/Write/Close path
- BenchmarkEncodeHandlerCorpus: full Encode.ServeHTTP middleware path
- Grid: 4 payloads × gzip levels 1/5/9 × zstd fastest/default/best
- Each subtest runs with 4 parallel workers; compare runs on MB/s and allocs/op

Conformance tests:
- Encoder contract: Reset, Flush, Close, and pool-style Reset-after-Close reuse
- Corpus HTTP encoding: Content-Encoding, Vary, ETag suffix, header stripping
- Response semantics: minimum_length, 304, HEAD, range, WebSocket bypass,
  If-None-Match rewrite, Cache-Control no-transform, content-type matcher rejection

modules/caddyhttp/encode/testdata/README.md

@@ -0,0 +1,7 @@
+These benchmark payloads are snapshots from caddyserver.com, fetched on
+2026-06-06:
+
+- `caddy_home.html`: https://caddyserver.com/
+- `caddy_config_http_servers.json`: https://caddyserver.com/api/docs/config/apps/http/servers
+- `caddy_asciinema_player.css`: https://caddyserver.com/resources/css/vendor/asciinema-player-3.6.1.css?v=378d6d0
+- `caddy_asciinema_player.js`: https://caddyserver.com/resources/js/vendor/asciinema-player-3.6.1.min.js?v=378d6d0