[S5][QA] Final P0 burn-down

**Sprint:** S5 (2026-08-10 to 2026-08-16) · **Build:** B11 · **Priority:** P0 · **Route:** `all routes`

Fix remaining P0 defects only — anything P1 or lower is deferred to post-GA patch releases. Every fix gets a targeted regression on its surrounding flow.

---
_Imported from the sprint tracker (2026-07-20)._