[Bug][Windows]: Treat horizontal document overflow as renderer verification failure

[rwang23]

Parent

None — standalone audit-remediation item.

What to build

Make Windows renderer verification fail when a skin introduces horizontal
document overflow, while allowing ordinary vertical scrolling for long
conversations.

Acceptance criteria

• Horizontal document overflow makes verification fail.
• Vertical-only overflow remains valid.
• The two scroll axes are tested independently.
• No installer, launcher, or official Codex binary behavior changes.

Blocked by

None.

Implementation

Draft implementation PR: #287. 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.