Key insight: tP (PV A-fragment base) used p_tmem_s.outer layout,
but P store used QK C-fragment composition layout. These diverge at hd>64.
Fix: tP now uses the same QK C-fragment composition layout (tStP_layout)
as the P store. PV A-fragment is derived from tP, so it automatically
uses the same layout. No double-offset since tP includes P offset.