docs: record the V2 sequencing — medium_changed emitted in V2b, tested in V4

docs/volume-manager-plan.md

@@ -49,6 +49,14 @@ harness once, not fat and the harness both.
 - Discrimination: a fixture transferring outside its assigned range must be
   refused; fails against a driver without the clamp.
 
+**Sequencing (as executed).** V2a landed the range clamp + its discrimination
+fixture (block-range) — the security mechanism is testable in isolation. V2b
+adds `medium_changed` and makes usb-storage a publisher that emits it on
+presence transitions, but its END-TO-END test (eject → medium_changed →
+unmount/remount) lands in V4 with the real consumer, the volume manager —
+rather than a throwaway subscriber fixture V3 would immediately replace. Same
+work, no duplicated scaffolding.
+
 ## V3 — the volume manager service
 
 New binary `system/services/volume-manager`, spawned by init, serving the