## ADDED Requirements

### Requirement: Camera frustum wireframe drawn every frame
For every `UsdGeomCamera` prim present in the current USD stage, the system SHALL draw a 3D wireframe representation into the viewport FBO each frame. The wireframe SHALL be drawn at `UsdTimeCode::Default()`. The wireframe SHALL consist of: (a) a small body box centred at the camera origin, (b) four lines from the camera origin to the corners of a near-clip display quad, and (c) a short up-arrow line indicating the camera's local Y axis.

#### Scenario: Camera wireframe appears when camera prim exists
- **WHEN** a `UsdGeomCamera` prim exists in the stage
- **THEN** a frustum wireframe is visible in the viewport at the camera prim's world-space position and orientation

#### Scenario: No wireframe when stage has no cameras
- **WHEN** no `UsdGeomCamera` prims exist in the stage
- **THEN** no camera wireframe is drawn

### Requirement: Wireframe size scales with viewport camera distance
The wireframe scale SHALL be proportional to the current viewport camera's look-at distance (`ViewportCamera::GetDist()`), clamped to a maximum of 50 world units, so that camera shapes maintain a consistent apparent screen size regardless of zoom level.

#### Scenario: Camera wireframe stays visible when zooming out
- **WHEN** the viewport camera is zoomed out (increasing distance)
- **THEN** the camera wireframe grows proportionally so it remains visible at roughly the same screen size

### Requirement: Three distinct visual states
The system SHALL draw camera wireframes using three colour states:
- **Inactive** (default): muted grey `(0.55, 0.55, 0.55, 0.85)`
- **Active** (the camera currently driving the viewport via the camera toolbar): cyan `(0.2, 0.9, 1.0, 1.0)`
- **Selected** (the camera prim is the current primary selection): yellow-orange `(1.0, 0.75, 0.1, 1.0)`

A camera that is both active and selected SHALL use the selected colour (selected takes priority).

#### Scenario: Active camera shown in cyan
- **WHEN** a USD camera prim is set as the active viewport camera via the camera toolbar
- **THEN** its wireframe is drawn in cyan

#### Scenario: Selected camera shown in yellow-orange
- **WHEN** a camera prim is the primary selected prim in the scene hierarchy
- **THEN** its wireframe is drawn in yellow-orange, overriding the cyan active colour

### Requirement: Camera list cached and refreshed on stage change
The system SHALL cache the list of `UsdGeomCamera` prim paths to avoid re-traversing the stage every frame. The cache SHALL be invalidated whenever `SetForceRefresh(true)` is called or the stage pointer changes.

#### Scenario: Cache invalidated on stage reload
- **WHEN** a new USD file is opened
- **THEN** the camera wireframe list reflects the new stage's cameras in the next frame
