## ADDED Requirements

### Requirement: Theme switch event taxonomy
The tracking taxonomy MUST define an event for theme switching.

The event name MUST be `theme_switch`.

The `theme_switch` event MUST include, at minimum:
- `target_id`
- `placement`
- `theme`

The event SHOULD include `prev_theme` when available.

The taxonomy MUST define the `target_id` namespace for theme switching as:
- `theme.switch.<theme>`

The taxonomy MUST define the `placement` value for the theme switcher notch as:
- `theme_notch`

#### Scenario: Theme switch target_id is deterministic
- **WHEN** a user selects `light` theme using the theme notch
- **THEN** the event is emitted with `target_id=theme.switch.light` and `placement=theme_notch`
