Replace the ...-with-tail variant actions with a pure tail_control module +
a docked-panel footer toggle (Off/Auto/Manual) read by CAPTURE_ITEM/TRACK.
Default None (byte-identical); Manual fixed 2s, in-memory session lifetime.
Tail mechanism unchanged; retired the variant ids.
Update the CLAUDE.md FX-scope invariant to item+track only; note master
removal in COMPLETED.md; drop master references from the tail spec and PLAN
Milestone T; realtime now taps the selected track.