Rebuild: edge:node --max-mbit caps a node's download

For workers on a shared line (poweredge-kantoor: 160 Mbit/s): the range
stream is read at most at the cap, TCP holds the coordinator back. The
cap is the node's, split over its processes (also EDGE_MAX_MBIT).
REBUILD.md: where workers run (connections set the speed; remote workers
take CPU off homelab).

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

lib/edge/index.ts

@@ -4,3 +4,4 @@ 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'
+export { throttle } from './throttle.js'

lib/edge/throttle.ts

@@ -0,0 +1,18 @@
+import { setTimeout as sleep } from 'node:timers/promises'
+
+/**
+ * Passes a byte stream on at most `bytesPerSecond` on average: a worker on a
+ * line it shares (`edge:node --max-mbit`) reads its range slower, and TCP
+ * holds the sender back. The first second's worth goes at once.
+ */
+export async function* throttle<T extends Uint8Array>(stream: AsyncIterable<T> | Iterable<T>, bytesPerSecond: number): AsyncGenerator<T> {
+  const started = Date.now()
+  let bytes = 0
+  for await (const piece of stream) {
+    yield piece
+    bytes += piece.byteLength
+    const due = started + ((bytes - bytesPerSecond) / bytesPerSecond) * 1000
+    const wait = due - Date.now()
+    if (wait > 0) await sleep(wait)
+  }
+}