Loop: an explicit enable, four named marks with grabbable caps, and the crossfade painted where it is actually heard

hasLoop becomes user-owned with the gestures as shortcuts onto it; no format change. START uses overlay/trace, not accent/primary, which is the waveform's own fill.

src/shell/instrument/CLAUDE.md

@@ -9,7 +9,7 @@ two small identity/helper headers this directory owns outright
 
 The pure engine/geometry core this shell wraps (`sampler_core`, `pitch_shift`,
 `sample_map`, `component_state_io`, `play_params.h`, `editor_geometry`, `sample_bands`,
-`sample_chrome`, `keyboard_strip`, `waveform_view`, `capture_browser`, `browser_scroll`,
+`sample_chrome`, `keyboard_strip`, `waveform_view`, `loop_marks`, `capture_browser`, `browser_scroll`,
 `param_slider`, `param_taper`, `trigger_seam`, `velocity_curve`, `embed_strip`, `knob_deck`,
 `deck_groups`, `deck_values`, `bake_hold`, `curve_popup`, `spline_edit`, `master_gain`,
 `limiter`, `meter_ballistics`, `reasampler_uid.h`) lives in `core/instrument/*` and