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` |

src/card-controller/view/modifiers/substream.ts

@@ -1,30 +1,38 @@
 import { View } from '../../../view/view';
 import { ViewModifier } from '../types';
 
-// The single write path for the camera-keyed `live.overrides` map (the read
-// path being `getStreamCameraID` in `view/substream`): sets a camera's
-// substream override, or clears it when `substreamID` is absent so the
-// camera's own stream is used. `cameraID` defaults to the selected camera.
-export class SubstreamViewModifier implements ViewModifier {
-  private _substreamID?: string;
-  private _cameraID?: string;
+interface SubstreamViewModifierOptions {
+  // The substream to engage. When absent, the override is cleared so the
+  // camera's own stream is used.
+  stream?: string;
 
-  constructor(substreamID?: string, cameraID?: string) {
-    this._substreamID = substreamID;
-    this._cameraID = cameraID;
+  // The camera whose override to write. Defaults to the selected camera.
+  camera?: string;
+}
+
+// The single write path for the camera-keyed `live.overrides` map (the read
+// path being `getStreamCameraID` in `view/substream`).
+export class SubstreamViewModifier implements ViewModifier {
+  private _options: SubstreamViewModifierOptions;
+
+  constructor(options?: SubstreamViewModifierOptions) {
+    this._options = options ?? {};
   }
 
   public modify(view: View): void {
-    const cameraID = this._cameraID ?? view.camera;
+    const cameraID = this._options.camera ?? view.camera;
     if (!cameraID) {
       return;
     }
-    if (!this._substreamID) {
+    // A stream equal to the camera itself is semantically "no substream";
+    // normalise it to a cleared override so the map doesn't carry self-
+    // referential entries.
+    if (!this._options.stream || this._options.stream === cameraID) {
       view.context?.live?.overrides?.delete(cameraID);
       return;
     }
     const overrides = view.context?.live?.overrides ?? new Map<string, string>();
-    overrides.set(cameraID, this._substreamID);
+    overrides.set(cameraID, this._options.stream);
     view.mergeInContext({ live: { overrides } });
   }
 }