Compute ETag from (mtime, size) to skip whole-file load per request

[ianegordon]

Note that this PR exchanges performance for accuracy. It changes ETag to be based on modification time & size rather than a proper SHA. This technique is also used by Apache and nginx.

This PR would also simplify updating the directory handler to adopt streaming as the file handler already does.

Summary

• Replace SHA-256-over-contents ETag with "<hex-mtime>-<hex-size>", matching nginx (ngx_http_set_etag in src/http/ngx_http_core_module.c) and Apache HTTPD's default FileETag MTime Size.
• FileHTTPHandler and DirectoryHTTPHandler no longer call Data(contentsOf:) solely to compute an ETag.
• Strong validator (no W/ prefix) so If-Range keeps working for FileHTTPHandler's Range path.

Closes TVT-290.

Test plan

• New HTTPCacheControlTests (6 tests): nil for missing file; quoted strong format with hex-hex inner; stable for unchanged file; differs on size change; differs on mtime change; collides when mtime+size match but contents differ (the property that distinguishes a metadata ETag from a content-hash ETag).
• swift test — 435 tests pass.
• swift build — clean, no warnings.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.74%. Comparing base (5295843) to head (8fd1c5d).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
FlyingFox/Sources/HTTPCacheControl.swift	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #212      +/-   ##
==========================================
- Coverage   92.75%   92.74%   -0.02%     
==========================================
  Files          70       70              
  Lines        3645     3653       +8     
==========================================
+ Hits         3381     3388       +7     
- Misses        264      265       +1     

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