fix(cua-driver): report encoder exit and shutdown timeout errors

[injaneity]

Summary

• Problem this solves: stop_recording hid encoder exit failures and shutdown timeouts behind the same generic “did not finalize” error.
• What changed: return the actual failure category and encoder exit code or timeout duration to callers, and retain that cause in get_recording_state.last_error. Add public-tool regression tests and execute them in Windows CI; Linux already runs them. Remove the standalone diagnostic workflow and process-comparison scripts. The existing wait loop returns success or the specific error directly, followed by shared stderr cleanup. No outcome enum, diagnostic bookkeeping, or generic process framework remains. The production method is four lines shorter than the original main-branch implementation; most of the remaining diff is durable regression coverage.

This is an error-reporting fix, not a fix for the intermittent Windows recording failure in #3713.

Related work

Refs #3713

RFC: not required for this correction to existing error reporting. No new tool, response schema, permission, or recording lifecycle contract is introduced.

Compatibility and risk

• User-visible, API/CLI/MCP, migration, permission, or platform impact: failed FFmpeg recordings now provide more specific error text through existing error fields. Windows and Linux FFmpeg recording paths are affected; native macOS ScreenCaptureKit is unchanged. No migration or permission changes. The three-second shutdown deadline and rejection of failed recordings remain unchanged.
• Risk and rollback: consumers matching exact error strings may need to accept the more specific messages. Reverting restores the old generic errors; there is no data migration. Small external encoder fixtures test failure reporting without mocking recorder internals or claiming successful video production.

Validation

• Focused tests and checks: two separate red → green cycles through public recording tools: encoder exit (ad0e78efa → 796c73593) and shutdown timeout (bf2e15727 → 3f6f12213). Both verify error responses, disabled recording state, no successful video path, and retained failure cause. At current head 5f088c831c5b7a41a8b52637fb729aeb18da9afd, 604 core unit tests and both unchanged integration tests pass locally; formatting and diff checks pass. The cleanup preserves the three-second deadline and strict acceptance, with the failure cause returned to callers and stderr retained in failure logs. Current-head platform CI is pending; the preceding head’s ordinary platform CI passed.
• Manual or platform evidence: focused Windows native run and Linux native run passed at preceding head f96f8f849bbc24cab4ecd5e92d25d64cbeab5343, before the shutdown refactor. These runs do not certify the current head. Existing preflight records real fixture activity and independently decodes the resulting video. Local fault-injection tests exercise the FFmpeg adapter, not native macOS recording.
• Known gaps or CI still required: the original intermittent finalization failure remains unresolved, and these passing runs do not establish its root cause or fix it. No full desktop matrix certification claimed. Earlier diagnostic evidence remains linked in Windows recorder intermittently reports failed FFmpeg finalization during native E2E preflight #3713 and PR history, not in the shipping tree. Current-head platform CI and affected native recertification are required before merge. The PR was already non-draft when this cleanup began; its readiness state was not changed by this update.

Contributor and release checks

• The PR is focused and the description matches the final diff.
• This change does not require an RFC, or the accepted RFC is linked above.
• Tests, documentation, and platform evidence are included or the gap is explained.
• The PR title is a Conventional Commit describing the production change.
• External contributor authorship is preserved, or no external contribution is included.
• If release-tracked files changed but this is intentionally non-releasing, the no-release label is applied.

Release intent: a Cua Driver patch for user-visible error reporting; the conditional no-release requirement does not apply. No external contribution is included.