Rebuild: a file's identity is computed once per batch, not per article

The file key and release identity (two SHA-1 hashes and the release rules)
were ~25% of the edge worker's time. edge:bench: boneless 100k → 137k
headers/s on one core, ath (2013) 70k → 143k.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/headers/index.ts

@@ -1,3 +1,3 @@
 export { parseSubject, type ParsedSubject } from './subject.js'
 export { fileKey, weakFileKey, counterFileKey, looksLikeFilename } from './file_key.js'
-export { segmentFromHeader, type IngestSegment, type OverviewHeader } from './segment.js'
+export { segmentFromHeader, type IngestSegment, type OverviewHeader, type IdentityCache } from './segment.js'