feat: Snapshots — 8 per preset save/recall

Adds snapshot system to Helix Stadium:

Backend:
- Snapshot data model (8 slots per preset, captures block bypass
  + params + master volume)
- Snapshot API endpoints: list, save, recall, rename, delete
- Integrated into preset serialization/deserialization
- Tracks current_snapshot in state / WebSocket broadcasts

Frontend:
- Camera icon in header bar with current snapshot number
- SnapshotPanel as slide-up overlay
- 8 snapshot slots in a horizontal row
- Tap to recall filled snapshot, tap empty slot to save
- Press-and-hold (600ms) to save current state
- Inline name editing on tap
- Discard Edits toggle
- Green flash indicator on save
- Delete button for active snapshot

src/presets/manager.py

@@ -19,7 +19,7 @@ import threading
 from pathlib import Path
 from typing import Optional
 
-from .types import Bank, FXBlock, FXType, MIDIMapping, Preset
+from .types import Bank, FXBlock, FXType, MIDIMapping, Preset, Snapshot
 
 logger = logging.getLogger(__name__)
 
@@ -70,6 +70,24 @@ def _preset_to_dict(preset: Preset) -> dict:
             }
             for key, mapping in preset.midi_mappings.items()
         },
+        "snapshots": {
+            str(slot): {
+                "name": snap.name,
+                "master_volume": snap.master_volume,
+                "chain": [
+                    {
+                        "fx_type": block.fx_type.value,
+                        "enabled": block.enabled,
+                        "bypass": block.bypass,
+                        "params": dict(block.params),
+                        "nam_model_path": block.nam_model_path,
+                        "ir_file_path": block.ir_file_path,
+                    }
+                    for block in snap.chain
+                ],
+            }
+            for slot, snap in preset.snapshots.items()
+        },
     }
 
 
@@ -97,6 +115,26 @@ def _preset_from_dict(data: dict) -> Preset:
             max_val=md.get("max_val", 1.0),
         )
 
+    snapshots = {}
+    for slot_str, snap_data in data.get("snapshots", {}).items():
+        snap_chain = []
+        for bd in snap_data.get("chain", []):
+            snap_chain.append(
+                FXBlock(
+                    fx_type=FXType(bd["fx_type"]),
+                    enabled=bd.get("enabled", True),
+                    bypass=bd.get("bypass", False),
+                    params=dict(bd.get("params", {})),
+                    nam_model_path=bd.get("nam_model_path", ""),
+                    ir_file_path=bd.get("ir_file_path", ""),
+                )
+            )
+        snapshots[int(slot_str)] = Snapshot(
+            name=snap_data.get("name", f"Snapshot {slot_str}"),
+            chain=snap_chain,
+            master_volume=snap_data.get("master_volume", 0.8),
+        )
+
     return Preset(
         name=data["name"],
         bank=data.get("bank", 0),
@@ -107,6 +145,7 @@ def _preset_from_dict(data: dict) -> Preset:
         routing_mode=data.get("routing_mode", "mono"),
         routing_breakpoint=data.get("routing_breakpoint", 7),
         tuner_enabled=data.get("tuner_enabled", False),
+        snapshots=snapshots,
     )
 
 
@@ -143,6 +182,9 @@ class PresetManager:
         # Auto-save debounce timer
         self._auto_save_timer: Optional[threading.Timer] = None
 
+        # Current snapshot slot (0 = none selected)
+        self._current_snapshot: int = 0
+
         logger.info("PresetManager root: %s", self._dir)
 
     # ── Public navigation API ───────────────────────────────────────────────

src/presets/types.py

@@ -89,6 +89,14 @@ class MIDIMapping:
     max_val: float = 1.0
 
 
+@dataclass
+class Snapshot:
+    """A saved snapshot of block states and parameters within a preset."""
+    name: str
+    chain: list[FXBlock] = field(default_factory=list)
+    master_volume: float = 0.8
+
+
 @dataclass
 class Preset:
     """A complete pedal preset — full signal chain state."""
@@ -104,6 +112,10 @@ class Preset:
     routing_mode: str = "mono"      # "mono" or "4cm"
     routing_breakpoint: int = 7     # chain index where pre/post split occurs
 
+    # ── Snapshots (8 per preset) ──
+    snapshots: dict[int, Snapshot] = field(default_factory=dict)
+    """Snapshot slots 1-8. Empty dict means no snapshots saved."""
+
 
 @dataclass
 class Bank: