Add ETag support for static files

[Copilot]

Suave.Files had no ETag support, so clients could only revalidate cached static files via If-Modified-Since (second-precision only).

Changes

• Weak ETag, on by default — Files.file now emits ETag: W/"{length:x}-{ticks:x}" derived from FileInfo.Length + LastWriteTimeUtc.Ticks. No hashing, no FileSystemWatcher, no extra I/O beyond the stat already needed for Last-Modified, so the opt-in concern raised in the issue doesn't apply.
• If-None-Match handling — ServeResource.resource short-circuits to 304 Not Modified before the If-Modified-Since branch. Supports * and comma-separated tag lists; comparison is opaque-string equality after trim (RFC 7232 §3.2 weak comparison — correct when the client echoes back what we sent). The ETag header is re-emitted on the 304 (which resets headers).
• ServeResource.resource signature — new getEtag : string -> string option parameter. Embedded-asset serving passes fun _ -> None, preserving existing behavior.
• Public helper — Files.fileEtag : string -> string exposed for callers who want the same value (e.g. custom pipelines).
• Tests — 6 cases in HttpFile.fs: emitted on 200, stable across calls, match → 304, 304 carries ETag, non-match → 200, * → 304.
• Docs — brief note in docs/files.md.

Example

// Nothing to configure; ETag is on for all file serving.
let app = Files.browseHome

// $ curl -I http://localhost:8080/index.html
// HTTP/1.1 200 OK
// ETag: W/"1a4-8dbf1c3a0b1a2e0"
// Last-Modified: Tue, 15 Jul 2026 14:20:00 GMT
//
// $ curl -I -H 'If-None-Match: W/"1a4-8dbf1c3a0b1a2e0"' http://localhost:8080/index.html
// HTTP/1.1 304 Not Modified
// ETag: W/"1a4-8dbf1c3a0b1a2e0"