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` |

docs/examples.md

@@ -77,10 +77,10 @@ automations:
         fullscreen: true
     actions:
       - action: custom:advanced-camera-card-action
-        advanced_camera_card_action: live_substream_on
+        advanced_camera_card_action: substream_on
     actions_not:
       - action: custom:advanced-camera-card-action
-        advanced_camera_card_action: live_substream_off
+        advanced_camera_card_action: substream_off
 ```
 
 ### Fullscreen with display mode and substream switching
@@ -117,7 +117,7 @@ elements:
         advanced_camera_card_action: display_mode_select
         display_mode: single
       - action: custom:advanced-camera-card-action
-        advanced_camera_card_action: live_substream_on
+        advanced_camera_card_action: substream_on
       - action: custom:advanced-camera-card-action
         advanced_camera_card_action: fullscreen
 automations:
@@ -126,7 +126,7 @@ automations:
         fullscreen: false
     actions:
       - action: custom:advanced-camera-card-action
-        advanced_camera_card_action: live_substream_off
+        advanced_camera_card_action: substream_off
       - action: custom:advanced-camera-card-action
         advanced_camera_card_action: display_mode_select
         display_mode: grid
@@ -637,10 +637,10 @@ cameras:
 automations:
   - actions:
       - action: custom:advanced-camera-card-action
-        advanced_camera_card_action: live_substream_on
+        advanced_camera_card_action: substream_on
     actions_not:
       - action: custom:advanced-camera-card-action
-        advanced_camera_card_action: live_substream_off
+        advanced_camera_card_action: substream_off
     conditions:
       - condition: interaction
         interaction: true