feat(prune): R3 guarded deletion — confirm-to-delete + panel button

Extend BANK_PRUNE_FOLDER from report-only to dry-run→confirm-with-manifest→
delete exactly the pure-core orphan set (fresh recompute, stale entries skipped).
Windows routes to Recycle Bin (SHFileOperation+FOF_ALLOWUNDO), else unlink.
New pure prune_button module + footer button dispatching the action id. No
ext-state write, no undo point (deletion is not REAPER-undoable).

src/prune_button.cpp

@@ -0,0 +1,39 @@
+#include "prune_button.h"
+
+// prune_button implementation — right-anchored button placement in the footer strip,
+// with a left-collision suppression rule. Trivially auditable arithmetic; the safety
+// property (a suppressed/empty button never claims a click) is a pure predicate tested
+// outside the DAW.
+
+namespace reasampler {
+
+ButtonRect computePruneButton(const FooterRect& footer, const PruneButtonSpec& spec) {
+    if (footer.width <= 0 || footer.height <= 0) return ButtonRect{};  // degenerate footer
+    if (spec.buttonWidth <= 0) return ButtonRect{};                    // nothing to place
+
+    // Right-anchored: right edge inset from the footer's right; width fixed.
+    const int right = footer.x + footer.width - spec.rightInset;
+    const int left = right - spec.buttonWidth;
+
+    // Suppress if the button would encroach past the reserved left inset (tail label room)
+    // or spill off the left of the footer entirely.
+    if (left < footer.x + spec.minLeftInset) return ButtonRect{};
+
+    // Vertically centred by the inset; clamp so a thin footer never yields a negative height.
+    int top = footer.y + spec.verticalInset;
+    int height = footer.height - 2 * spec.verticalInset;
+    if (height <= 0) {
+        top = footer.y;
+        height = footer.height;
+    }
+
+    return ButtonRect{left, top, spec.buttonWidth, height};
+}
+
+bool hitTestPruneButton(int px, int py, const ButtonRect& button) {
+    if (button.empty()) return false;  // suppressed button claims nothing
+    return px >= button.x && px < button.x + button.width &&
+           py >= button.y && py < button.y + button.height;
+}
+
+} // namespace reasampler