feat(audio-sequences): beat phase sync and aligned playback

Add bar-phase tracking, audio reset/anchor APIs, BPM holdover, beat-phase
sequence switching, sync-phase endpoint, and sample sequence data.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/models/zone.py

@@ -22,7 +22,7 @@ class Zone(Model):
     """Preset layout row (stored in ``db/zone.json``); legacy storage was ``tab.json`` / Tab.
 
     Optional ``content_kind`` on a row: ``\"presets\"`` (preset tiles only) or ``\"sequences\"``
-    (sequence tiles only). Omitted or unknown => both (legacy behaviour).
+    (sequence tiles only). Legacy rows without ``content_kind`` are inferred on load.
     """
 
     def __init__(self):
@@ -43,6 +43,12 @@ class Zone(Model):
             if "preset_group_ids" not in doc or not isinstance(doc.get("preset_group_ids"), dict):
                 doc["preset_group_ids"] = {}
                 changed = True
+            if "sequence_ids" not in doc or not isinstance(doc.get("sequence_ids"), list):
+                doc["sequence_ids"] = []
+                changed = True
+            if not self._normalized_content_kind(doc):
+                doc["content_kind"] = self._infer_content_kind(doc)
+                changed = True
         if changed:
             self.save()
 
@@ -53,6 +59,41 @@ class Zone(Model):
         kind = doc.get("content_kind")
         return kind if kind in ("presets", "sequences") else None
 
+    @staticmethod
+    def _preset_ids_in_doc(doc):
+        if not isinstance(doc, dict):
+            return []
+        flat = doc.get("presets_flat")
+        if isinstance(flat, list):
+            return [str(x) for x in flat if x is not None and str(x).strip()]
+        presets = doc.get("presets")
+        if not isinstance(presets, list) or not presets:
+            return []
+        if isinstance(presets[0], str):
+            return [str(x) for x in presets if x is not None and str(x).strip()]
+        if isinstance(presets[0], list):
+            out = []
+            for row in presets:
+                if isinstance(row, list):
+                    out.extend(str(x) for x in row if x is not None and str(x).strip())
+            return out
+        return []
+
+    @classmethod
+    def _infer_content_kind(cls, doc):
+        kind = cls._normalized_content_kind(doc)
+        if kind:
+            return kind
+        seq_ids = [
+            str(x).strip()
+            for x in (doc.get("sequence_ids") or [])
+            if x is not None and str(x).strip()
+        ]
+        preset_ids = cls._preset_ids_in_doc(doc)
+        if seq_ids and not preset_ids:
+            return "sequences"
+        return "presets"
+
     def _enforce_content_kind_invariants(self, doc):
         """Presets-only zones hold no sequences; sequences-only hold no preset tiles."""
         kind = self._normalized_content_kind(doc)
@@ -95,7 +136,8 @@ class Zone(Model):
             return False
         patch = data if isinstance(data, dict) else {}
         self[id_str].update(patch)
-        self._enforce_content_kind_invariants(self[id_str])
+        if "content_kind" in patch:
+            self._enforce_content_kind_invariants(self[id_str])
         self.save()
         return True