feat: add M6 insert — place selected bank sample at edit cursor

InsertMedia-driven placement reading the bank panel selection, undo-wrapped.
Native length by default; conform-to-tempo is an explicit second action, never
a silent stretch. Pure mode-bit logic (insert_plan) unit-tested.

src/bank_panel.h

@@ -1,8 +1,8 @@
 #pragma once
 // bank_panel — the docked grid window (M5, Wave A). REAPER-facing shell: it owns
 // a SWELL dialog docked via DockWindowAddEx, and paints the current project's
-// bank as a grid of LICE-drawn waveform thumbnails. Read-only this wave: it NEVER
-// inserts into the arrange or mutates the project/bank (CONTEXT.md §load-bearing
+// bank as a grid of LICE-drawn waveform thumbnails. The panel itself NEVER inserts
+// into the arrange or mutates the project/bank (CONTEXT.md §load-bearing
 // principle). Audition / multi-select / keyboard nav are Wave B.
 //
 // The header is REAPER-free as practical: main.cpp drives the panel through these
@@ -10,6 +10,9 @@
 // All SWELL / LICE / PCM_source use is confined to bank_panel.cpp. The pure
 // layout math and cache keys live in bank_grid (unit-tested outside the DAW).
 
+#include <string>
+#include <vector>
+
 namespace reasampler {
 
 class ReaSamplerSession;
@@ -29,6 +32,17 @@ void bankPanelToggle();
 // checked-state (toggleaction) so REAPER shows a tick next to the menu entry.
 bool bankPanelIsOpen();
 
+// The stable ids of the currently-selected samples, in bank (insertion) order.
+// Empty when nothing is selected or the panel has never opened. This is the clean
+// seam the `insert` action reads to know WHAT to place — it returns ids (not grid
+// indices) so the caller resolves against the live bank and is unaffected by the
+// panel's internal index bookkeeping. READ of panel state only; no mutation.
+//
+// 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.
+std::vector<std::string> bankPanelSelectedSampleIds();
+
 // 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.