Emit a Date header on all responses (RFC 9110 §6.6.1)

[ianegordon]

Summary

Two related conformance fixes for HTTP date handling:

1. IMF-fixdate formatting — HTTP dates are now formatted per RFC 9110 §5.6.7 via a new shared HTTPDate formatter: pattern "EEE, dd MMM yyyy HH:mm:ss 'GMT'". The previous pattern ("EEE, d MMM yyyy HH:mm:ss zzz") emitted single-digit days, violating the ABNF (day = 2DIGIT), and relied on locale behaviour for the zone name instead of the literal GMT. HTTPCacheControl and the file/directory handlers now share this single formatter.

2. Date header on every response — RFC 9110 §6.6.1: "An origin server with a clock … MUST generate a Date header field in all 2xx (Successful), 3xx (Redirection), and 4xx (Client Error) responses, and MAY generate a Date header field in 1xx (Informational) and 5xx (Server Error) responses." HTTPConnection.sendResponse now injects an IMF-fixdate Date when the response doesn't already carry one (unconditionally across status classes — conformant and simpler). Handler-supplied Date values are never overwritten. Since sendResponse is the single server egress, framework-authored responses (404 unhandled, 500 handler-throw, timeout) are covered without running user code.

Tests

All 456 package tests pass.

• HTTPDateTests: exact output for both RFC example dates ("Sun, 06 Nov 1994 08:49:37 GMT", "Tue, 15 Nov 1994 08:12:31 GMT"), zero-padded-day case, format→parse round-trip.
• HTTPConnectionTests: generated well-formed Date on a 200; byte-exact test proving a supplied Date is preserved verbatim.
• HTTPServerTests: wire-level assertions that 404-unhandled, 500-handler-throw, and timeout responses carry a well-formed IMF-fixdate Date.

Notes

• Per-response cost measured at ~1.4 µs per format call on Apple Silicon (~1.4% of one core at 10k req/s). If that ever matters, the usual mitigation is a once-per-second cached string; not done here to keep the change minimal.
• DateFormatter thread safety is documented for Apple platforms (iOS 7+/macOS 10.9+); swift-corelibs-foundation documents no equivalent guarantee. The shared-static-formatter pattern already existed in HTTPCacheControl; this PR extends its use to every response.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.87%. Comparing base (ac24c58) to head (5b3298d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
+ Coverage   92.83%   92.87%   +0.03%     
==========================================
  Files          71       72       +1     
  Lines        3743     3748       +5     
==========================================
+ Hits         3475     3481       +6     
+ Misses        268      267       -1     

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[swhitty]

looks great, thanks