7800a5152b1f3b4e15d7063a314ce5383c495b7e  AX: In isolated tree mode, an empty text field's text markers point into its placeholder

Classified as not security-relevant.
Triage note: textRuns() now returns empty for placeholder text (isPlaceholderText checks the shadow host's placeholderElement); an accessibility correctness fix so placeholder text isn't exposed as control text, no memory-safety or security-boundary bug.
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 (+4.87), null_guard (+1.2), surgical (+1.3)