feat: pattern-based generators from real track analysis, RPP structure fixes, randomization

- Reverse-engineer drum patterns from 2 real reggaeton tracks with librosa
- Create patterns.py with extracted frequency data (kick/snare/hihat positions)
- Rewrite rhythm.py with pattern-bank generators (dembow, dense, trapico, offbeat)
- Rewrite melodic.py with section-aware generators and humanization
- Add weighted random sample selection in SampleSelector (top-5 pool)
- Add generate_structure() with randomized templates and energy variance
- Fix RPP structure: TEMPO arity (3→4 args), string quoting for empty strings
- Rewrite quick_drumloop_test.py with correct REAPER ground truth format
- Add scripts/analyze_examples.py for reverse engineering audio tracks
- Add --seed argument for reproducible generation
- 72 tests passing

src/selector/__init__.py

@@ -16,6 +16,7 @@ from __future__ import annotations
 
 import json
 import os
+import random
 from pathlib import Path
 from typing import Optional
 from dataclasses import dataclass, field
@@ -306,10 +307,27 @@ class SampleSelector:
         matches.sort(key=lambda m: m.score, reverse=True)
         return matches[:limit]
 
-    def select_one(self, role: str, **kwargs) -> Optional[dict]:
-        """Select the single best matching sample."""
-        results = self.select(role=role, limit=1, **kwargs)
-        return results[0].sample if results else None
+    def select_one(
+        self,
+        role: str,
+        seed: Optional[int] = None,
+        **kwargs,
+    ) -> Optional[dict]:
+        """Select one sample using weighted random from top-5 candidates.
+
+        The top-5 candidates are selected with weights [5, 4, 3, 2, 1],
+        favoring higher-scored results while allowing variation across calls.
+        Pass seed for reproducible output.
+        """
+        if seed is not None:
+            random.seed(seed)
+        results = self.select(role=role, limit=5, **kwargs)
+        if not results:
+            return None
+        candidates = results[:5]
+        weights = [5, 4, 3, 2, 1][: len(candidates)]
+        selected = random.choices(candidates, weights=weights, k=1)[0]
+        return selected.sample
 
     def get_roles(self) -> list[str]:
         """Get all available roles and their counts."""