Cover fork readiness lifecycle invariants

[sjmiller609]

Summary

• guarantee seed Envoy cleanup on every snapshot-start-page exit and cover process/bootstrap teardown
• cover the applied-marker gate across each post-fork readiness probe
• name the five-second fork identity response margin alongside the shared apply timeout

Performance

20 sequential headless Firecracker forks per version (8 vCPU), measuring the fork request through the successful fork-identity response and verifying public CDP returned 200 on every run. The Envoy fixture used a static listener to isolate local process startup/readiness from xDS and network variance.

version	fork-to-ready p50	fork-to-ready p95	identity handoff p50	identity handoff p95
before 759635d	293 ms	492 ms	69 ms	96 ms
after #324 (0503e9f)	1,564 ms	1,708 ms	1,340 ms	1,498 ms

All runs passed (20/20 per version). The after p95 leaves 28.3 seconds of headroom in the 30-second API timeout and 23.3 seconds inside the wrapper's 25-second readiness deadline.

But it's OK that this is slower because we are migrating off of envoy and the hot pools avoid hitting this case. Also we have another tier of idle pool which we can start filling with fanout.

Testing

• cd server && go vet ./...
• cd server && go test -race $(go list ./... | grep -v /e2e$)
• cd server && make build
• git diff --check main...HEAD

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Note

Medium Risk
Touches fork handoff timing and when the applied marker is written; incorrect gating could stall or falsely signal fork readiness to the API.

Overview
Tightens fork-identity readiness so the applied marker is only written when every post-fork probe succeeds, and documents the 5s response margin next to ApplyTimeout in forkidentity.

writeForkIdentityAppliedMarker centralizes that gate; readiness polling is split into waitProbesReady so tests can assert per-probe behavior (cdp, chromedriver, envoy) without a full boot.

Snapshot start page seed Envoy teardown is guaranteed on all exit paths via a defer, with cleanupSeedEnvoyWith for unit tests of stop + bootstrap removal.

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