fix(cua): soften idle backstop after visible progress

[danielgwilson]

Summary

• give screenshot/wait idle turns a bounded recovery window when the UI visibly progresses, so loading or transition frames do not carry an actor straight into the idle backstop
• keep that forgiveness finite so animated pixels, synthetic turn counters, or endlessly changing state cannot avoid the idle backstop forever
• add an idle-specific recovery hint before giving up on screenshot/wait-only loops
• add a blocked trace notice with last material action, recent action labels, and screenshot reference when a backstop fires

Closes #243.

Public Boundary

• This change does not include PII, PHI, secrets, keys, tokens, raw private transcripts, private screenshots, or private customer/patient data.
• Examples, fixtures, personas, screenshots, and artifact snippets are synthetic or redacted.
• Product-specific private context stays outside this repository or in synthetic adapter fixtures only.

Proof

• pnpm exec vitest run tests/computer-use.test.ts — 30 passed on Node 24.12.0
• source ~/.nvm/nvm.sh && nvm exec 22.14.0 pnpm exec vitest run tests/computer-use.test.ts — 30 passed
• pnpm exec vitest run tests/cua-actor-lab.test.ts -t "a gave_up in-process run is a failed lane" — 1 passed
• pnpm exec vitest run tests/cua-actor-lab.test.ts — 51 passed
• pnpm exec vitest run tests/computer-use.test.ts tests/cua-actor-lab.test.ts — 81 passed
• pnpm typecheck — passed
• pnpm build — passed
• pnpm public-surface:scan — passed
• git diff --check -- src/computer-use.ts tests/computer-use.test.ts — passed

Remaining Gaps

• Full pnpm release:check is left to CI for this PR. The first revision reproduced an OOM in tests/cua-actor-lab.test.ts; this revision fixes that path locally with the full file passing.