feat: fix sample variety per section and reorganize sample library

- Fix compose.py to select different samples per section instead of one per role
- Add select_many() to SampleSelector for diverse sample selection
- Migrate 862 samples from scattered dirs to libreria/samples/{role}/
- Rename files with consistent convention: {role}_{key}_{bpm}_{character}_{hash}.wav
- Add migrate_library.py script with dry-run and verification
- Backup original index as sample_index_pre_migration.json
- 72 tests passing

scripts/compose.py

@@ -235,13 +235,11 @@ def build_section_tracks(
     # Build one track per role
     tracks: list[TrackDef] = []
 
+    # Track used sample IDs per role for diversity
+    used_sample_ids: dict[str, list[str]] = {}
+
     for role, role_cfg in roles.items():
         sample_role = ROLE_TO_SAMPLE_ROLE.get(role, role)
-        generator_name = role_cfg.get("notes_template", "")
-
-        # Select sample for this role
-        sample_match = selector.select_one(role=sample_role, key=key, bpm=bpm)
-        sample_path = sample_match.get("original_path") if sample_match else None
 
         # Collect clips for each section
         section_clips: list[ClipDef] = []
@@ -251,6 +249,20 @@ def build_section_tracks(
             vel_mult = section.energy
             vol_mult = section.energy
 
+            # For audio roles, select a different sample per section
+            sample_path = None
+            if role in AUDIO_ROLES:
+                exclude = used_sample_ids.get(role, [])
+                diverse_results = selector.select_diverse(
+                    role=sample_role, n=1, exclude=exclude, key=key, bpm=bpm
+                )
+                if diverse_results:
+                    sample = diverse_results[0]
+                    sample_path = sample.get("original_path")
+                    sample_id = sample.get("file_hash", "")
+                    if sample_id:
+                        used_sample_ids.setdefault(role, []).append(sample_id)
+
             if role in ROLE_RHYTHM_GENERATORS:
                 gen_name = ROLE_RHYTHM_GENERATORS[role]
                 note_dict = get_notes(gen_name, section.bars, velocity_mult=vel_mult)
@@ -281,12 +293,12 @@ def build_section_tracks(
                 gen_fn = ROLE_MELODIC_GENERATORS[role]
                 note_list = gen_fn(key=key, bars=section.bars, velocity_mult=vel_mult)
                 midi_notes = melodic_to_midi(note_list)
+                # Melodic roles use MIDI instruments — no audio_path needed
                 clip = ClipDef(
                     position=sec_offset * 4.0,
                     length=section.bars * 4.0,
                     name=f"{section.name.capitalize()} {role.capitalize()}",
                     midi_notes=midi_notes,
-                    audio_path=sample_path,
                 )
                 section_clips.append(clip)