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/index.ts

@@ -1,4 +1,6 @@
-export { RangeIndexer, type EdgeBatch, type ReleaseChunk, type DroppedChunk, type ProbeCandidate, type CandidateKind, type RuleCount, type Fragment, type RangeIndexerOptions } from './range_indexer.js'
+export { RangeIndexer, chunkOf, candidatesOf, releaseKeyOf, type EdgeBatch, type ReleaseChunk, type DroppedChunk, type ProbeCandidate, type CandidateKind, type RuleCount, type Fragment, type RangeIndexerOptions } from './range_indexer.js'
 export { encodeFrame, decodeFrames, isKeepalive, KEEPALIVE, type Frame } from './stream.js'
 export { toWire, fromWire, type WireBatch } from './wire.js'
 export type { Lease } from './lease.js'
+export { mergeFragments } from './merge.js'
+export type { EdgeSegment, FileMeta } from './scan.js'