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/edge/range_indexer.ts

@@ -1,7 +1,7 @@
 import { createHash } from 'node:crypto'
 import type { Overview } from '../nntp/index.js'
 import { parseXrefGroups } from '../nntp/index.js'
-import { segmentFromHeader, type IngestSegment } from '../headers/index.js'
+import { segmentFromHeader, type IdentityCache, type IngestSegment } from '../headers/index.js'
 import { FileAggregator, type AggregatedFile, isArchiveStart, isLaterVolume, sevenZipVolume } from '../files/index.js'
 import { RuleSet, isRandomFile, ownerOf, type Rules } from '../rules/index.js'
 import { encodeChunk, type ChunkFile } from '../chunks/index.js'
@@ -104,6 +104,8 @@ export class RangeIndexer {
   private rules: RuleSet
   private batch: EdgeBatch
   private counts = new Map<string, RuleCount>()
+  /** File identities seen in this batch (cleared with each take, so it stays small). */
+  private identities: IdentityCache = new Map()
   private readonly idle: number
   private readonly edge: number
 
@@ -127,7 +129,7 @@ export class RangeIndexer {
         this.count('crosspost', owner, 1, 0)
         continue
       }
-      const segment = segmentFromHeader(header, group, articleNumber)
+      const segment = segmentFromHeader(header, group, articleNumber, this.identities)
       if (segment) segments.push(segment)
       else this.count('obfuscated', '', 1, 0)
     }
@@ -145,6 +147,7 @@ export class RangeIndexer {
   /** What was found since the last take, and a fresh batch. */
   take(): EdgeBatch {
     this.judgeReleases()
+    this.identities.clear()
     const out = this.batch
     out.counts = [...this.counts.values()]
     this.counts.clear()