fix(cu): preserve delivered action state

[hqhq1025]

Summary

Follow-up to the non-blocking executor findings on merged #910.

• return outcome_unknown when AX or CDP text delivery cannot be confirmed by readback
• preserve outcome_unknown when a delivered semantic action cannot produce its required fresh screenshot
• store observations only after screenshot normalization succeeds, avoiding unreachable FIFO entries
• update model-facing copy for the uniquely targeted Electron CDP text path

Safety boundary

Pre-dispatch target identity, freshness, occlusion, physical-input, and compatibility-input checks are unchanged. This PR only prevents delivered actions from being mislabeled as safely retryable capture failures.

Verification

• Computer Use full suite: 127/127
• Computer Use typecheck
• Runtime build
• git diff --check

[hqhq1025]

@Astro-Han CI typecheck/test/e2e is fully green. This addresses the delivered-state and unreachable-observation findings from merged #910: AX/CDP text readback mismatch and post-dispatch fresh-capture failure now return outcome_unknown; failed screenshot normalization no longer consumes FIFO capacity; model copy reflects the unique Electron CDP text path. Computer Use full suite passed 127/127 locally. Ready for review.

[Astro-Han]

Approved. I did not find a P0 or P1. This is the follow-up round for #910, so the remaining P2/P3 items are non-blocking.

1. [P2] Preserve delivered state for every fresh-observation failure after a semantic action. The new catches at cua-driver-backend.ts:1916-1932 and :1984-2005 handle only CuaDriverCaptureError, which covers screenshot normalization. A later get_window_state failure, page-identity lookup error, compressor exception, or request-lifecycle error still rejects or becomes an ordinary capture/service failure after the action was delivered. I reproduced this through the page-identity seam: the AX click reached the driver once, the fresh identity read threw, and runSemantic rejected instead of returning outcome_unknown. Convert any post-delivery verification failure to the delivered-but-unverified result.

2. [P2] Text writes still lose delivery uncertainty when the verifier itself fails. The AX and CDP changes cover a parseable readback mismatch, but not a failing readback request. After native set_value succeeds, a failed snapshotTarget is still reported as capture_failed; after CDP insert_text succeeds, an inspection error or exception follows the same path. Those cases must also return outcome_unknown so the model does not safely retry text that may already be present.

3. [P3] Update the module-level keyboard invariant. The comment at cua-driver-backend.ts:18-24 still says Electron targets fail before dispatch, while the implementation and corrected tool description now support uniquely targeted Electron CDP text.