1623de98773e0e3c0173a0004f16ecf836cd7997  AX: Detect and report form validation errors a page never announced or associated with a field

Classified as not security-relevant.
Triage note: Adds AccessibilityFormErrorDetectionEnabled preference, new AXFormActivityMonitor class and AX notification plumbing; net-new accessibility feature behind a flag, not a fix.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), bounds_guard (+4.32), null_guard (+2.59), clamp_saturate (+1.8), area:Source/WebCore/html (+2.0), large_churn (-2.0)