fix(sync): gate on index content hash, not just ETag — CDN edges rotate ETags

Second gated sync observed ~70% of roots re-checked 40 minutes apart with
byte-identical index bodies: Apple's CDN rotates ETags across edges without
content changes, and every rotation fell through to a full per-page check
of that root. The body hash (schema_meta root_index_hash:<slug>) is now the
authoritative signal; a rotated ETag over identical content skips the
root's page checks and adopts the fresh ETag immediately.