feat(midi): strum chords with preserved note ends and undo (phase 99)

docs/CREATIVE_WORKFLOWS.md

@@ -82,3 +82,14 @@ 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.
+
+## Chord strumming
+
+Select MIDI clips and use **Edit → MIDI tools → Strum chords...**. Enter spacing
+in ticks (1–960) and direction (0 ascending, 1 descending). Notes sharing a source
+onset and MIDI channel within each clip form a chord. Equal pitches use stable
+note-ID order. Each successive note starts one spacing later, while its end,
+pitch, velocity and channel remain unchanged. Single notes remain unchanged.
+Any resulting zero/negative duration rejects the entire edit; Undo restores the
+whole selection. This edits source notes, including notes hidden by clip trims.
+It does not generate an arpeggiator or schedule live input.