Rebuild: the merger stops when a pass over the file keys decides nothing

Waiting files counted as work, so the merge loop re-read them for its
whole 30 s budget every minute: Postgres used ~a core even without
workers. Now ~4% at rest, and storing a boneless range costs Postgres
~0.7–1.9 s of CPU (it was 33 s with the loop running). Chunk payloads
are stored EXTERNAL (zstd already; no pglz attempt).

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

commands/coordinator_run.ts

@@ -48,14 +48,20 @@ export default class CoordinatorRun extends BaseCommand {
         }
         if (Date.now() - mergedAt >= MERGE_EVERY_MS) {
           mergedAt = Date.now()
-          // Rounds until nothing more can be decided, or MERGE_BUDGET_MS.
+          // Rounds until a whole pass over the file keys decides nothing more, or MERGE_BUDGET_MS.
+          // (Going on while only waiting files were left kept Postgres busy all the time.)
           const totals = { completed: 0, incomplete: 0, waiting: 0 }
+          let decidedThisPass = 0
           while (!stopping && Date.now() - mergedAt < MERGE_BUDGET_MS) {
             const merged = await FragmentMerger.run()
             totals.completed += merged.completed
             totals.incomplete += merged.incomplete
             totals.waiting = merged.waiting
-            if (merged.files === 0) break
+            decidedThisPass += merged.completed + merged.incomplete
+            if (merged.wrapped) {
+              if (decidedThisPass === 0) break
+              decidedThisPass = 0
+            }
           }
           if (totals.completed || totals.incomplete) this.logger.info(`Fragments: ${totals.completed} files completed, ${totals.incomplete} incomplete, ${totals.waiting} waiting.`)
         }