[plan] Fix October broken sessions root causes

_Automatically composed from the parent epic at dispatch time._

Split by Architecture boundary: 7 independent coder children (C1–C7) in one phase "Root-cause fixes", no depends_on. R1c needs no child because Task 9264 already shipped it. AC-D2 stays deferred to Finish; it sits in C3, and C1/C2 point to it. Acceptance tightened with the critic's minor cases. Memory: none applied.