remove dead action_buttons module (superseded by action_bar in Phase L)

Deletes src/action_buttons.{h,cpp} and tests/test_action_buttons.cpp;
removes the static library, test target, and reaper_reasampler link
entry from CMakeLists.txt; scrubs all dangling comment references.

src/component_geometry.h

@@ -2,8 +2,8 @@
 // component_geometry — the REAPER-free, LICE-free geometry + hit-test math for the shared
 // drawing kit's generic components (Phase L, L1): a button box, a slider's track/handle,
 // and a list row. These are the kit-level primitives that DON'T already have a pure owner:
-// bank_grid / mode_switch / tab_strip / action_buttons / prune_button stay the source of
-// truth for the surfaces THEY own; this module carries only the new, reusable component
+// bank_grid / mode_switch / tab_strip / prune_button stay the source of truth for the
+// surfaces THEY own; this module carries only the new, reusable component
 // shapes the kit's drawButton / drawSlider / drawListRow draw against.
 //
 // Why pure (CLAUDE.md §load-bearing split, DS-1 caution): even where the draw shell reuses
@@ -47,8 +47,8 @@ bool hitTestBox(int px, int py, const KitBox& box);
 //
 // A button drawn inside a host cell, inset by a uniform padding so it reads as a raised
 // control rather than a full-bleed fill (the kit's drawButton draws the micro-gradient
-// surface inside this box). Distinct from prune_button/action_buttons, which own their
-// OWN placement within their strips — this is the generic "given a cell, where's the
+// surface inside this box). Distinct from prune_button, which owns its OWN placement
+// within its strip — this is the generic "given a cell, where's the
 // button" helper for new kit consumers.
 struct KitButtonBox {
     KitBox box;