[Bug][macOS]: Renderer readiness accepts non-visible nodes

[rwang23]

Parent

None — standalone audit-remediation item.

What to build

Harden macOS renderer readiness so a nonzero DOM box alone does not establish
visibility. During a document swap, transient Runtime.evaluate failures must
continue retrying only within the existing bounded deadline.

Acceptance criteria

• Detached, opacity-zero, collapsed or CSS-hidden, content-visibility:hidden, offscreen, and checkVisibility() === false anchors are not accepted as visible.
• Transient renderer-evaluation errors do not terminate the verify loop before its existing deadline.
• Regression tests cover the visibility and retry boundaries.
• Official Codex installation and the loopback CDP boundary remain unchanged.

Blocked by

None.

Implementation

Draft implementation PR: #283. This issue was created after the audit so the
independently reviewable remediation has a dedicated tracking record. It should
close only when the linked PR merges.