Fix embed use-after-free, restore golden fixture + refs tests, drop dead note_entry

src/shell/instrument/CLAUDE.md

@@ -10,7 +10,7 @@ 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`,
-`note_entry`, `param_slider`, `trigger_seam`, `velocity_curve`, `embed_strip`, `knob_deck`,
+`param_slider`, `trigger_seam`, `velocity_curve`, `embed_strip`, `knob_deck`,
 `curve_popup`, `master_gain`, `reasampler_uid.h`) lives in `core/instrument/*` and
 `core/wire` and is documented there — this directory consumes it but does not own it.