ci(nests): defer cross-stack interop CI; document manual run

Removes the `hang-interop` + `browser-interop` jobs from
`build.yml` (added in commit `21947bc5` after a 10/10 stability
sweep × 22 tests = 220/220 hard-pass). Cold-cache cost is
~10 min hang + ~13 min browser; warm-cache critical-path add
is ~6 min via the parallel browser job. Most PRs don't touch
audio / MoQ / QUIC, so paying that cost on every PR is
net-negative for the change-pattern the repo sees.

Adds `nestsClient/tests/README.md` covering:
- when to run (changes to nip53 / moq-lite session / audio
  pipeline / MoqLiteNests* / ReconnectingNests* / :quic /
  the sidecars themselves);
- quick-start gradle commands for hang-only, browser-only,
  combined;
- prerequisites + first-run cache costs;
- all the configuration knobs (incl. the
  `-DnestsHangInteropTraceRelay=true` trace-capture switch
  added during the routing investigation);
- known limitations (hot-swap browser soft-pass,
  framesPerGroup pin-vs-prod gap, I7 cycle 2 truncation);
- a 4-step debug recipe for triaging a flaking scenario.

Plan updates:
- `2026-05-07-t16-closure-roadmap.md` — Priority 3 marked
  ⏸ DEFERRED instead of ✅ CLOSED, pointing at the new README.
- `2026-05-07-cross-stack-interop-ci-gating.md` — status
  changed to ⏸ DEFERRED; YAML shape preserved verbatim in the
  plan for the next revisit.
- `2026-05-06-cross-stack-interop-test-results.md`,
  `2026-05-06-cross-stack-interop-test-gap-matrix.md` — CI
  integration § rewritten to "manual-run only" + README link.

The trace-capture instrumentation in `NativeMoqRelayHarness`
stays in place; it's useful for future flake triage even
without CI.

nestsClient/plans/2026-05-07-t16-closure-roadmap.md

@@ -67,22 +67,23 @@ HangInteropTest with their current soft-pass assertions intact.
 
 **Acceptance bar met.** 5/5 sweep with hard assertions.
 
-## Priority 3 — `2026-05-07-cross-stack-interop-ci-gating.md` ✅ CLOSED
+## Priority 3 — `2026-05-07-cross-stack-interop-ci-gating.md` ⏸ DEFERRED (manual run only)
 
-> **Closed 2026-05-07** by commit `21947bc5`. Both
-> `hang-interop` and `browser-interop` jobs landed in
-> `.github/workflows/build.yml`, gated on `lint`,
-> `ubuntu-latest`, 30 min timeout, with cached cargo + bun +
-> Playwright Chromium. Path-tweaked from the original removed
-> shape because the browser harness moved from
-> `nestsClient-browser-interop/` to
-> `nestsClient/tests/browser-interop/` (commit `bd7b166f`).
-
-**Acceptance bar met.** 10/10 sweep BUILD SUCCESSFUL × 22 tests
-= **220/220 pass** (~5m 28s steady state per sweep on the agent
-rig). The first 2 weeks of post-merge CI runs still need a
-maintainer to monitor flake rate (per the plan's "≥ 95% green
-rate" gate); pull the jobs again if it falls below.
+> **Briefly wired then reverted, 2026-05-07.** Commit `21947bc5`
+> re-added both jobs to `build.yml`; verification gave 10/10
+> sweep × 22 tests = 220/220 hard-pass on the agent rig. A
+> subsequent maintainer review judged the cold-cache cost
+> (~10 min hang, ~13 min browser) too high for the change-pattern
+> (most PRs don't touch audio / MoQ / QUIC) and removed the jobs.
+>
+> The suites stay green locally and are intended to run before
+> merging any change that touches the audio / moq-lite / `:quic`
+> stack. Documentation: [`nestsClient/tests/README.md`](../tests/README.md)
+> covers when/how/prerequisites/debugging.
+>
+> The CI-gating plan is kept around in case the wallclock cost
+> calculus changes (e.g., a much faster runner, or a smaller
+> opt-in subset of the suite that runs in <2 min).
 
 ## Independent track — `2026-05-07-framespergroup-production-rerun.md`