fix(browser): bound forced-close teardown to one deadline

[seonghobae]

Partial implementation of #28. This Draft adds a shared-deadline forced-close teardown observer for the pinned Chromium Agent Task lane.

Falsifiable defect and repair lineage

The forced-close pass previously waited for the browser-root identity and sampled Chromium process set through two sequential helpers. Each helper could consume the full PROCESS_EXIT_TIMEOUT_SECONDS budget, allowing one nominal teardown phase to approach twice the intended deadline.

Test-only head 2abcfacf8181ba51d905026e7fe085cb4a0bd149 required one combined observer with separate root/process-set evidence and one monotonic deadline. CI 31694255612, Rust-contract job 94428272804, reached the intended RED because _wait_for_linux_process_teardown was absent. Production then added the narrow shared-deadline observer and routed forced-close teardown through it; root/process-set outcomes remain distinct and fail closed.

Fresh stack truth

Current contributor head is exact 962e179b53ed0342fde5b50c3deac0e7454b0d4f. The live prerequisite branch test/agent-task-forced-close-failure-teardown-evidence is exact 48435b9d7e74ae932f03248c41833c684b1be411.

A fresh compare reports the current child 10 commits ahead / 0 behind that live prerequisite, with merge base exactly 48435b9d7e74ae932f03248c41833c684b1be411. The intervening production reconciliation deliberately preserves the prerequisite's WebDriver diagnostic redaction/narrow-exception hardening while retaining this child's one-deadline teardown semantics. GitHub reports the PR open, Draft, and mergeable against the exact live parent.

Exact-current evidence

On unchanged exact head 962e179b53ed0342fde5b50c3deac0e7454b0d4f:

• CI run 32773795820: success;
• Rust contracts job 97580050351: Python repository contracts, canonical formatting, locked workspace/all-target check, full tests, strict Clippy, and rustdoc success;
• Production coverage job 97580050162: success, explicitly checking out exact SHA 962e179b53ed0342fde5b50c3deac0e7454b0d4f and reporting owned production functions, lines, regions, and branches all exactly 100% covered;
• exact coverage artifact 9537217522, exact-coverage-962e179b53ed0342fde5b50c3deac0e7454b0d4f, uploaded ZIP SHA-256 98b0cf9f9f365446024e90577d74c64e1e73da0316e526dd0b2da092890845b1;
• Manifest V3 Compatibility run 32773795778: success; and
• all current inline review threads are resolved. Formal review inventory contains automated COMMENTED reviews only; no qualifying independent approval is inferred.

No exact-head SAST or Security Scan is claimed because the exact workflow inventory exposes only CI and Manifest V3 Compatibility for this head. Predecessor checks/reviews are not transferred.

Truth and authority boundary

This branch proves a bounded Linux/pinned-Chromium forced-close teardown observation for an exact root PID/start-time identity and already sampled Chromium PID/start-time set under one deadline. It does not establish cgroup/task ownership, discover later-created processes, prove OS-wide orphan absence, implement cross-platform supervision, or turn the CI runner into a product browser API.

Protected-main AGENTS.md remains authoritative. This scheduled writer does not merge, self-approve, force-push, alter workflows/rulesets/secrets, tag, release, or publish.

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[devin-ai-integration]

Devin Review found 1 new potential issue.

[devin-ai-integration]

📝 Info: Two-deadline teardown remains in the non-forced-close pass

The non-forced-close pass still calls _wait_for_linux_process_identity_exit then _wait_for_linux_process_identity_set_exit in sequence (run_mv3_compatibility.py), each with its own full PROCESS_EXIT_TIMEOUT_SECONDS. This is the same double-budget shape the PR fixes for forced-close, left unchanged here. Out of scope for this partial PR, but a candidate for the same combined-waiter treatment.

(Refers to this code)

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