Rebuild: the fragment merger

Files cut by a range's edge are put together once the ranges around them
are done (coordinator:run, every minute): a complete file is judged by the
rules and written as a release chunk with probe candidates; one still
missing parts while both neighbouring ranges are done is dropped and
counted (incomplete/edge). A neighbour not planned yet counts as not done,
except before the group's oldest article. BatchStore.writeReleases is the
shared write path.

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

lib/edge/range_indexer.ts

@@ -280,7 +280,7 @@ export class RangeIndexer {
 }
 
 /** The release key; a file without a recognisable release becomes a release of its own. */
-function releaseKeyOf(s: EdgeSegment): string {
+export function releaseKeyOf(s: EdgeSegment): string {
   return s.file.release?.key ?? createHash('sha1').update(`file\n${s.fileKey}`).digest('hex')
 }
 
@@ -293,7 +293,7 @@ function postedOf(segments: EdgeSegment[]): number | null {
   return min
 }
 
-function chunkOf(key: string, files: AggregatedFile<EdgeSegment>[]): ReleaseChunk {
+export function chunkOf(key: string, files: AggregatedFile<EdgeSegment>[]): ReleaseChunk {
   const first = files[0].segments[0].file
   const chunkFiles: ChunkFile[] = files.map((f) => {
     const m = f.segments[0].file
@@ -339,7 +339,7 @@ function chunkOf(key: string, files: AggregatedFile<EdgeSegment>[]): ReleaseChun
  * volume, of its par2 index, of a lone random-named file, and a 7z set's
  * last volume (its listing is in the tail). At most a few per release.
  */
-function candidatesOf(key: string, files: AggregatedFile<EdgeSegment>[]): ProbeCandidate[] {
+export function candidatesOf(key: string, files: AggregatedFile<EdgeSegment>[]): ProbeCandidate[] {
   const out: ProbeCandidate[] = []
   let tail: { volume: number; file: AggregatedFile<EdgeSegment> } | null = null
   for (const f of files) {