refactor: Unify substream actions into substream_{on,off} (#2497)

## Summary

- Collapse `live_substream_{on,off,select}` into a unified
`substream_{on,off}` pair, symmetric with `call_{start,end}`.
- Rename the `camera` field on the former `live_substream_select` to
`stream` (it always was a stream ID).
- Add optional `camera` field to both new actions for targeting a
non-selected base camera.
- YAML configs are migrated automatically; URL bookmarks must be updated
by hand.

## Migration

### Cycling between camera and substream (toggle button)

Before:
```yaml
tap_action:
  action: custom:advanced-camera-card-action
  advanced_camera_card_action: live_substream_on
```

After:
```yaml
tap_action:
  action: custom:advanced-camera-card-action
  advanced_camera_card_action: substream_on
```

### Selecting a specific substream

Before:
```yaml
tap_action:
  action: custom:advanced-camera-card-action
  advanced_camera_card_action: live_substream_select
  camera: camera.front_door_hd
```

After:
```yaml
tap_action:
  action: custom:advanced-camera-card-action
  advanced_camera_card_action: substream_on
  stream: camera.front_door_hd
```

### Turning the substream off

Before:
```yaml
tap_action:
  action: custom:advanced-camera-card-action
  advanced_camera_card_action: live_substream_off
```

After:
```yaml
tap_action:
  action: custom:advanced-camera-card-action
  advanced_camera_card_action: substream_off
```

### URL querystrings (manual update required)

| Before | After |
| --- | --- |
|
`?advanced-camera-card-action.live_substream_select=camera.front_door_hd`
| `?advanced-camera-card-action.substream_on=camera.front_door_hd` |

tests/card-controller/view/modifiers/substream.test.ts

@@ -6,7 +6,7 @@ describe('SubstreamViewModifier', () => {
   it('should write the override for the selected camera', () => {
     const view = createView({ camera: 'camera' });
 
-    new SubstreamViewModifier('substream').modify(view);
+    new SubstreamViewModifier({ stream: 'substream' }).modify(view);
 
     expect(view.context?.live?.overrides?.get('camera')).toBe('substream');
   });
@@ -28,7 +28,10 @@ describe('SubstreamViewModifier', () => {
       context: { live: { overrides: new Map([['camera', 'substream']]) } },
     });
 
-    new SubstreamViewModifier('other-substream', 'other-camera').modify(view);
+    new SubstreamViewModifier({
+      stream: 'other-substream',
+      camera: 'other-camera',
+    }).modify(view);
 
     expect(view.context?.live?.overrides?.get('other-camera')).toBe('other-substream');
     expect(view.context?.live?.overrides?.get('camera')).toBe('substream');
@@ -40,15 +43,26 @@ describe('SubstreamViewModifier', () => {
       context: { live: { overrides: new Map([['other-camera', 'other-substream']]) } },
     });
 
-    new SubstreamViewModifier(undefined, 'other-camera').modify(view);
+    new SubstreamViewModifier({ camera: 'other-camera' }).modify(view);
 
     expect(view.context?.live?.overrides?.get('other-camera')).toBeUndefined();
   });
 
+  it('should clear the override when the stream equals the camera itself', () => {
+    const view = createView({
+      camera: 'camera',
+      context: { live: { overrides: new Map([['camera', 'substream']]) } },
+    });
+
+    new SubstreamViewModifier({ stream: 'camera' }).modify(view);
+
+    expect(view.context?.live?.overrides?.get('camera')).toBeUndefined();
+  });
+
   it('should no-op for a view without a camera', () => {
     const view = createView({ camera: null });
 
-    new SubstreamViewModifier('substream').modify(view);
+    new SubstreamViewModifier({ stream: 'substream' }).modify(view);
 
     expect(view.context).toBeNull();
   });