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/insert.h

@@ -0,0 +1,59 @@
+#pragma once
+// insert — placement of bank samples into the arrange (M6). REAPER-facing shell:
+// it reads the bank_panel's current selection, resolves each selected sample's
+// file, and drops it into the arrange at the edit cursor via InsertMedia, wrapped
+// in an undo block.
+//
+// THE INTENDED PLACEMENT PATH (CONTEXT.md §load-bearing principle): capture NEVER
+// auto-inserts; `insert` is the deliberate, user-invoked placement act, so it IS
+// allowed and expected to add items to the arrange. It must only ever run from its
+// own action — never from a capture path.
+//
+// Non-destructive to the bank: insert references the bank file (adds an arrange
+// item pointing at it); it never modifies the bank, the bank files, or ext state.
+// No SILENT time-stretch: conform-to-tempo is an explicit opt-in on the request,
+// defaulting OFF (native length). See insert_plan for the mode-bit computation.
+//
+// The header is SDK-free: all REAPER API use lives in insert.cpp. The pure
+// mode-bit arithmetic lives in insert_plan (unit-tested outside the DAW).
+
+#include "insert_plan.h"
+
+namespace reasampler {
+
+class ReaSamplerSession;
+
+// What one insert action does. Carries the InsertMedia options (target track +
+// tempo-conform choice) so the two action variants (native-length vs
+// conform-to-tempo) differ only by this struct — no divergent code paths.
+struct InsertRequest {
+    InsertOptions options;  // defaults: new track, no conform, native length
+};
+
+// The outcome of an insert action, for the caller to log to the console.
+enum class InsertStatus {
+    Ok,               // one or more samples inserted
+    NoSelection,      // the panel had no selection — a no-op (not an error)
+    NoProject,        // no saved project, so no resolvable bank dir — no-op
+    NothingResolved,  // a selection existed but no sample resolved to a file
+};
+
+struct InsertResult {
+    InsertStatus status = InsertStatus::NoSelection;
+    int inserted = 0;   // how many samples were actually placed
+    int skipped  = 0;   // selected-but-unresolvable/unreadable samples skipped
+};
+
+// Runs the insert: reads bank_panel's selection, resolves each sample against the
+// current project dir, and inserts them AT THE EDIT CURSOR in bank order, advancing
+// the cursor so multiple samples lay end-to-end. The whole placement is wrapped in
+// a single Undo_BeginBlock2 / Undo_EndBlock2 so one undo removes the entire insert.
+// `session` supplies the live bank the selected ids resolve against.
+//
+// Multi-select behavior: each selected sample is inserted sequentially at the
+// then-current edit cursor; InsertMedia advances the cursor to the end of the
+// inserted media, so N samples lay contiguously left-to-right. Single-select is the
+// N==1 case of the same path.
+InsertResult runInsert(ReaSamplerSession* session, const InsertRequest& request);
+
+} // namespace reasampler