fix(child-sessions): close cancelled acquisitions and retain current progress evidence

[tt-a1i]

Problem

A post-#426 current-main audit at eaf470b reproduced child startup ownership and progress-projection defects:

• Interrupting ordinary Pi child acquisition while the session factory is pending allows the late child to bind extensions after cancellation, with no abort/shutdown/dispose owner.
• Cancelling Workflow during git worktree add leaves an empty checkout/branch because the cancellation throw precedes its cleanup try/finally.
• With twenty 16-KiB tool results followed by an error, the 256-KiB transcript projection retains older results and drops the newest failure despite its first/newest retention contract.
• Tool previews split an entire growing output buffer although only the first nonempty line is displayed; a bounded scan is under measurement.

Scope and validation

Deliver one bounded repair PR: preserve Pi lifecycle, user configuration, permissions and canonical messages; no new configuration or orchestration framework. Add deterministic acquisition/cancellation regressions, mixed-size latest-evidence tests, and an execution-shaped preview performance probe. Validate check/full tests and two independent reviews. Keep existing viewport/hydration/shell/terminal-artifact work in its own PRs (#327, #319, #423, #386).

Runtime provenance: this is an audit of current main in an isolated worktree. The user's configured Pi still reports openpi-main-runtime at c8f2c13; installed behavior is not automatically changed by a source merge.

[tt-a1i]

Repair is ready in #429, left open for review.

Canonical investigation and bounded preview measurements.

Local bun run check and standard bun run test passed: 1446 Node tests, one platform skip, 30 Vitest tests. Two independent reviews are clean after also preserving isolated startup worktrees for pure/mixed interruptions when quiescence is unknown. Deterministic regressions cover late factory/binding, real Git post-checkout cancellation, dirty evidence, latest error retention and preview behavior. No user runtime, role files or settings changed. Exact-head remote CI passed on 342ac927736b64d12c8953e0c4c932eedecb3451: Node 22, Node 24 and Windows. No merge performed for this PR.