feat(bank_panel): B4 vertical-split UI — LICE tab strip, id-keyed bank ops, move/copy drag

Pool grid on top, LICE-drawn named-banks tab strip below with overflow-scroll
(new pure tab_strip seam, unit-tested). Full-height toggles, unmistakable
active-bank readout distinct from the shown tab, tab context menu
(activate/rename/delete/evacuate/create) with rich confirm-on-non-empty-delete,
and move/copy via menu + drag with drop-highlighting. Fold-in: deserialize
auto-disambiguates duplicate folded bank names instead of rejecting the book.

src/bank_panel.h

@@ -43,8 +43,23 @@ bool bankPanelIsOpen();
 // Note: the panel's selection is cleared on a bank change (capture / project
 // load), so a returned id always names a sample present in the current bank at
 // the moment of the call; the caller still tolerates an absent id gracefully.
+//
+// Phase B4 (vertical split): the selection lives in whichever REGION the user last
+// interacted with (the pool grid on top or a named-bank grid below), which is NOT
+// necessarily the active/capture-target bank. The returned ids therefore name
+// samples in the FOCUSED region's displayed bank — the bank the user visibly
+// selected in. Pair with bankPanelSelectedSourceBankId() to know which bank those
+// ids belong to (the move/copy source).
 std::vector<std::string> bankPanelSelectedSampleIds();
 
+// The bank id the current selection belongs to — the displayed bank of the region
+// the user last interacted with (pool region -> the pool id; named-banks region ->
+// the shown tab's bank id). This is the SOURCE bank for a move/copy of the current
+// selection, and it is distinct from the active/capture-target bank (active ≠ shown).
+// Returns the pool id when nothing is selected or the panel has never opened (a safe
+// default source). READ of panel state only; no mutation.
+std::string bankPanelSelectedSourceBankId();
+
 // Requests a repaint if the bank changed since the last paint (generation bump).
 // Cheap when nothing changed. Driven by the timer so a capture / project load is
 // reflected without the panel diffing the bank itself.