Write page-cache files atomically to avoid partial cache reads

[mehul0810]

Finding

Page-cache metadata and body files are written directly to their final paths with file_put_contents().

Evidence

• src/Modules/Cache/PageCache.php write_cache_meta() writes JSON metadata directly to the final meta path.
• src/Modules/Cache/PageCache.php write_cache_body() writes HTML directly to the final body path.
• Readers call read_cache_meta() and read_cache_body() directly against those same final paths.

Impact

The request lock prevents duplicate writers for the same key, but it does not prevent concurrent readers from seeing partially-written files. A busy site can serve truncated HTML or ignore freshly-written metadata/body pairs during cache regeneration.

Suggested fix

Write metadata and body to temporary files in the same cache directory, then atomically rename them into place. Write the body before the metadata so a fresh metadata file never points at a missing or partial body file.

Acceptance criteria

• Cache writes use temp-file-plus-rename semantics.
• Readers never observe partial body or metadata content.
• Existing cache key/file naming remains backward compatible.

[mehul0810]

Completed in PR #89 against release/1.6.0. Page-cache body and metadata writes now use atomic file writes.