Rebuild: end-to-end fixes — spool accounting, keep-alive frames, resume point, overlapped worker

End to end (workstation, recent a.b.boneless ranges from the spool): a 2M
range in 10–12 s, 166k–202k headers/s per worker process (edge:bench on
one core: 215k).

- Prefetcher: spooled_at was set against a total counted from the range's
  resume point, which shrinks as the worker advances: the last chunks were
  never fetched and the worker waited. Now counted against the whole
  range, only new chunks counted, and leased ranges are always checked
  against the files. Planned ranges ahead are bounded (it had spooled ever
  more); the spool size is measured every 30 s (a stat per file).
- The stream sends an empty frame every 20 s while waiting for a chunk
  (Node's fetch gave up on a quiet body after 5 minutes).
- A range taken over midway: the first batch starts at the resume point
  (it started at the range's start and was refused).
- edge:node inflates the next chunk on the thread pool while indexing the
  current one, and doesn't wait for a post before indexing on (posts stay
  in order; a failed post stops the range).

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

app/controllers/node_controller.ts

@@ -4,10 +4,12 @@ import { setTimeout as sleep } from 'node:timers/promises'
 import Leases, { LeaseLost } from '#services/leases'
 import BatchStore, { StaleBatch } from '#services/batch_store'
 import Spool from '#services/spool'
-import { encodeFrame, fromWire, type WireBatch } from '#lib/edge/index'
+import { KEEPALIVE, encodeFrame, fromWire, type WireBatch } from '#lib/edge/index'
 
 /** How long the stream waits for a chunk the prefetcher hasn't spooled yet. */
 const CHUNK_WAIT_MS = 15 * 60 * 1000
+/** While waiting, an empty frame this often: an HTTP client gives up on a quiet body (Node's after 5 minutes). */
+const KEEPALIVE_MS = 20_000
 
 /**
  * The edge workers' API (REBUILD.md, parts 1–2): a lease, the range's
@@ -34,8 +36,14 @@ export default class NodeController {
     async function* frames() {
       for (const span of spans) {
         const waitUntil = Date.now() + CHUNK_WAIT_MS
+        let quietSince = Date.now()
         while (!(await Spool.has(rangeId, span))) {
           if (Date.now() > waitUntil) throw new Error(`Chunk ${span.from}–${span.to} of range ${rangeId} wasn't spooled in time`)
+          if (Date.now() - quietSince >= KEEPALIVE_MS) {
+            quietSince = Date.now()
+            yield encodeFrame(KEEPALIVE)
+            await Leases.heartbeat(rangeId, nodeId)
+          }
           await sleep(500)
         }
         yield encodeFrame({ ...span, data: await Spool.read(rangeId, span) })