fix(gateway): preserve paced qualification traffic

openspec/changes/correct-gateway-qualification-evidence/design.md

@@ -19,9 +19,12 @@ The qualification driver already reaches the production Apollo-to-QUIC path, but
 ## Decisions
 
 - Reuse the existing source-boundary shaper, preserve source order unless explicit reorder is enabled, and limit catch-up to one media serialization interval. Record the fixed-seed applied-delay standard deviation separately from the jitter observed after ordered traversal.
+- Drive processing sends at the configured source rate while a separate public-client receive loop validates ordered payload delivery. Use cooperative scheduling with a bounded high-resolution final wait in the parent driver so sub-millisecond packet spacing does not depend on host sleep granularity.
 - Assign stable source sequence identifiers and retain per-stage counts so injected loss, provider/FEC drop, queue replacement, QUIC failure, and client miss are disjoint.
 - Reuse the established gateway child-test pattern for the actual gateway server; the Apollo fixture and QUIC client remain in the parent driver. A token-protected loopback test control endpoint starts and stops bounded child-owned recording and returns aggregate stage state.
 - Stream queue, processing, and pacing samples from the production `sendMedia` boundary to child-owned raw evidence. Sample child `RUSAGE_SELF`, Go heap, allocations, and goroutines once per second with independent per-run baselines.
+- Buffer at most 4,096 child-owned timing samples before the gzip writer; drain every sample before recording stops and backpressure on sustained writer overload instead of dropping evidence or compressing synchronously in the media loop.
+- Use the existing reviewed 64-packet qualification bound for native video (about 7.5 ms at 80 Mbps) after public-path counters proved the 16-packet queue replaced 1–5 clean-path units during ordinary scheduler pauses. Keep audio at 16 packets and retain latest-unit replacement at both bounds.
 - Measure clock overhead as the median elapsed time per read across 1,000 batches of 100 monotonic reads and record that method.
 
 ## Risks / Trade-offs
@@ -29,4 +32,5 @@ The qualification driver already reaches the production Apollo-to-QUIC path, but
 - [Ordered release suppresses some delivered jitter] → Retain both the applied fixed-seed delay distribution and the separately observed ordered-traversal jitter.
 - [Stage attribution double-counts a unit] → Record one terminal outcome per source sequence and validate accounting equality.
 - [Process sampling perturbs qualification] → Use bounded low-rate samples and include the sampling method in evidence.
+- [The source driver consumes CPU for precise pacing] → Keep it in the parent process excluded by the gateway-only resource sampler, and yield cooperatively until the final 50 microseconds.
 - [Shared private runners cannot sustain the reviewed 20/50/80 Mbps gates] → Run the complete verifier on the registered on-demand xhigh runner; the frozen qualification remains authoritative for the normative duration.