Add ETag support and fix conditional request handling in ServeDir

[jlizen]

Closes #324, closes #411

Motivation

ServeDir only supports time-based conditional requests today. This causes real problems: if you roll back a deploy, files get an older mtime and clients keep getting 304s for stale content. HTTP dates also have 1-second granularity, so rapid redeploys within the same second are invisible to If-Modified-Since.

Solution

Generate a strong ETag from file metadata (mtime with nanosecond precision + file size) and implement the full RFC 9110 §13.2.2 precondition evaluation order:

1. If-Match → 412 (enables safe range request resumption)
2. If-Unmodified-Since (when If-Match absent) → 412
3. If-None-Match → 304
4. If-Modified-Since (when If-None-Match absent) → 304

The ETag format is "<hex_secs>.<hex_nanos>-<hex_size>". Similar to what nginx does (<mtime>-<size>) but with nanosecond precision. No content hashing, no extra I/O; we already call metadata() on every request.

304 responses now include ETag and Last-Modified headers per RFC 9110 §15.4.5 (previously they were bare, which was a spec violation).

No new public API surface. All new types are pub(super). The ETag is always-on, matching the existing Last-Modified behavior.

Testing

Covers: ETag presence on 200, If-None-Match producing 304 (exact match, wildcard, weak prefix), If-Match producing 412 (non-matching, weak prefix rejected by strong comparison), RFC precedence (ETag conditions override time-based when both present), and validator headers on 304 responses.