* perf(relay): stop ACK boundary scans at first pending boundary
* test(relay): pin PTY source boundary cleanup and guard ascending sends
The early-`break` in advanceCredit is only correct while sentBoundaries is
inserted in ascending sentEndSu order. Turn that implicit invariant into a
throw at the sole live write site (commitPtySourceSend), and assert the
post-state directly instead of inferring it from an iteration budget:
- assert the surviving boundary set after the 1,023-ACK benchmark
- cover the jump-ahead cumulative ACK that must delete many boundaries in
one pass (the case an over-eager `break` would get wrong)
- cover the settleReservedPtySourceAck -> advanceCredit entry point
- drop an arithmetically-implied assertion and CI benchmark log noise
* perf(relay): reclaim ACK boundaries with a monotone cursor
The early-break Set scan still rebuilt a Set iterator per ACK, so V8 walked
delete tombstones and the drain stayed superlinear; the visit-count test could
not see it because it stubbed sentBoundaries with a generator over a private
Set. Replace the Set with an ascending boundary list plus a monotone cursor,
assert the real structure, and add a benchmark over the shipped code.
* test(relay): enforce ascending sent-boundary inserts in the collection
Move the ascending-order precondition into PtySourceSentBoundaries.add so
both insert sites are covered, and assert per-ACK span reclamation in the drain.
* test(relay): collapse ledger test record accessors into getDeliveryRecord
Rebase onto #17490 left two structurally identical internals accessors
(getCursorRecord, getBoundaryRecord); one typed accessor covers both.
