Replay: a covered or off-viewport element falls through to pixel rungs #1479

backends/playwright_backend.py:2027-2048 returns None for a present but covered element, which is the same as a missing one. Resolution then falls through to template and OCR rungs and a raw mouse click, which can land on the overlay.

Proposed fix: return a typed "exists but not actionable" refusal when the locator matched exactly one element, and don't fall through to pixel rungs.

Found by the realism audit (docs/audits/REALISM_AUDIT.md, #1467), rank 17.