Relocate TailSetting into ReaSamplerSession; persist serializes it to
a new forever-stable "tail_setting" ext-state key (load-per-project,
save-on-save). Footer scroll-wheel adjusts Manual length in 250ms
steps; label now shows "Tail: Manual X.Xs". Pure step/label/JSON
round-trip covered by tests.
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.