Generates a weak ETag from a hash of the response body and evaluates the RFC 9110 conditional-request preconditions in Apache's ap_meets_conditions order via ConditionalRequest: If-Match -> If-Unmodified-Since -> If-None-Match -> If-Modified-Since.

Outcomes:

• 304 Not Modified — GET/HEAD whose validators say "unchanged".
• 412 Precondition Failed — failed If-Match / If-Unmodified-Since, or a matched If-None-Match on a non-GET/HEAD method.
• otherwise the original response, with an ETag header on GET/HEAD.

Usage in app.php: $app->addMiddleware(new \ZealPHP\Middleware\ETagMiddleware());

Streaming responses (SSE, stream(), Generator yield) are skipped — they have no buffered body to hash.

ETag derivation across paths (audit gap H7): ZealPHP emits weak ETags everywhere, but the validator's input depends on how the response is produced — exactly as Apache differs between static and dynamic content:

• Buffered / dynamic responses (this middleware): W/"xxh3(body)".
• Zero-copy file sends (Response::sendFile()): W/"mtime-size" (stat-based, no body hash — Apache never hashes a static file body either). The two paths are mutually exclusive per response: this middleware bails on streaming responses and on an empty buffered body (sendFile() streams), so it never overwrites a stat-based ETag. Because both forms are weak, a URL that switches serving path produces a cache miss (full 200), never a corrupt 304 — weak comparison can't false-match across the two namespaces.