feat(audio): crossfade overlapping clips in one undoable edit (phase 98)

docs/CREATIVE_WORKFLOWS.md

@@ -71,3 +71,14 @@ are ignored. Each accepted tempo is an undoable edit and stops playback before
 preparation; tapping is unavailable while recording. New/Open resets tap history.
 The estimator uses monotonic control-thread time, with no audio-callback clock
 access or allocation. Tap timing is not a beat detector or MIDI clock sync.
+
+## Audio crossfades
+
+Select two partially overlapping audio clips on the same track and choose
+**Edit → Audio tools → Crossfade two selected clips**. The earlier clip fades
+out across the overlap while the later clip fades in. Gains, positions, source
+files and the opposite fades are preserved. Equal boundaries, containment,
+adjacency, cross-track selections and MIDI clips reject without partial edits.
+This is a linear tick-based crossfade using existing clip envelopes; it does
+not move clips, create overlaps or offer equal-power curves. Discrete sample
+endpoints retain the engine's existing one-sample boundary convention.