perf: desktop cold-start runner small follow-ups (#5997) #6003

Desktop cold-start runner: small follow-ups from its review (#5997, #6000)

The desktop cold-start A/B runner (scripts/perf/desktopColdStart.ts, workflow "Desktop Cold Start") works. Its A/A on 9c6e4c3e3b gave a half-width of 0.96% (run 38015899054), so it gates the #5971 PRs. Its lane and reviewers left these Low findings. None blocks a measurement today.

1. Playwright's uncaught timeout stack does not name the launch index.
2. SIGTERM sent to the node process alone does not stop a run.
3. tsx comes in only through the e2e package, not as a direct dependency.
4. The base tree's Electron can download before a bad head tree is rejected.
5. The entry guard throws if a module imports the script without a script path. Nothing does today.
6. base_sha must contain 🤖 perf: first-load JS report and app-shell-ready mark #5981 (the xum:app-shell-ready mark), or every launch times out waiting for it. The workflow header documents this only for head_sha.

Owner: the perf owner. Trigger: fix item 6 in the next T3 PR that edits the workflow, and the others when the runner is next changed or one of them costs a run.

Generated with xum • Model: anthropic:claude-opus-5-5 • Thinking: high