feat(midi): import Standard MIDI Files with transactional undo (phase 104)

docs/MIDI_WORKFLOWS.md

@@ -60,3 +60,25 @@ are cropped and notes outside the trim are omitted. Trailing silence remains.
 Export does not edit the project or undo history. Audio clips reject. Only note
 messages are exported: instruments, opaque plugin data, controller maps,
 automation and audio are excluded. Export is unavailable during recording.
+
+## Import MIDI files
+
+File → Import MIDI (also in the sequencer Create menu) adds a clip at the cursor
+on the selected non-bus track, or creates a track when none is selected. Import
+is one undoable prepared edit; malformed files, capacity errors and failed
+backend preparation retain the old project and IDs.
+
+The reader supports Standard MIDI File types 0 and 1 with PPQN timing, including
+running status and velocity-zero note-offs. File tracks merge into one clip;
+channels and velocities remain. Timing rounds to 960 ticks per quarter note,
+with positive sub-tick notes given one tick. Trailing silence remains. The project
+tempo is retained: tempo maps, time signatures, CC/sustain, program changes,
+pitch bend, SysEx and other metadata are not imported. This is a notes workflow,
+not full-song or instrument reconstruction.
+
+Limits are 1 MiB per file, 64 file tracks, 65,536 parsed events and 4,096 notes
+within the project's total note capacity. SMPTE timing and type 2 reject explicitly.
+Unterminated notes, zero source duration and malformed events reject rather than
+silently dropping notes. Unmatched note-offs are harmless and ignored. Overlapping
+same-pitch notes pair FIFO within each file track/channel. Project save/reload
+uses the existing format; no format migration or extra dependency is required.