Fix full-page cache handling for HTTP HEAD requests

Problem

The full-page cache treats HEAD as cacheable, but a cache hit emits the cached HTML body and exits. HTTP HEAD responses must have the same headers as GET without a message body. Sending the body wastes bandwidth and can confuse monitors, crawlers, and reverse proxies that use HEAD for freshness checks.

Evidence

• src/Modules/Cache/PageCache.php:962-966 allows both GET and HEAD requests through the cacheability gate.
• src/Modules/Cache/PageCache.php:161-176 emits $html for both a fresh hit and a stale hit without checking the request method.
• The existing page-cache tests do not cover fresh, stale, or lock-stale HEAD responses.

Scope

Correct HEAD behavior in the local page-cache path while preserving cache keys, HIT/STALE headers, invalidation, and GET behavior.

Implementation plan

1. Introduce one request-method helper and use it consistently in cacheability and cache-response delivery.
2. On fresh, stale, and stale-lock hits, send the same cache state headers as GET but return no body for HEAD; include a correct Content-Length only when it is safe to provide it.
3. Decide and document the miss behavior so a HEAD request never writes or emits a body unexpectedly.
4. Keep internal regeneration and preload requests on their existing GET path.
5. Add focused PHPUnit coverage for fresh HIT, STALE, and STALE-LOCK HEAD requests and for an unaffected GET HIT.

Acceptance criteria

• A cached HEAD request returns the expected X-Perform-Cache state and an empty body.
• A GET request for the same cache key still returns the cached body unchanged.
• HEAD never creates a cache entry or changes cache freshness state as an unintended side effect.
• Fresh, stale, and lock-contention paths are all covered by regression tests.

Validation

• composer test
• composer phpstan
• PHPCS on changed PHP files
• Isolated WordPress runtime proof: warm a page with GET, then compare GET and HEAD status, headers, and response byte count.

Non-goals and risk

• Do not broaden page-cache invalidation; issue #197 and PR #199 own lifecycle invalidation.
• Do not alter cache keys, TTLs, Cloudflare behavior, or public settings.
• Preserve HTTP compatibility for proxies that issue HEAD requests.

Start in src/Modules/Cache/PageCache.php, especially lines 161-176 for cache-hit delivery and 962-966 for the cacheability gate. Trace fresh, stale, stale-lock, miss, and internal regeneration paths, then add focused PHPUnit coverage for HEAD and unaffected GET behavior. Done means HEAD preserves cache-state headers with no body while GET, invalidation, and cache freshness behavior remain unchanged.