feat: implement overlay architecture with IOverlayLayer interface and telemetry overlay

app/overlay/overlay_layer.py

@@ -0,0 +1,54 @@
+"""IOverlayLayer — interface for pluggable overlay layers drawn on CameraView."""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+
+from PySide6.QtCore import QRect
+from PySide6.QtGui import QPainter
+
+
+class IOverlayLayer(ABC):
+    """
+    Interface for a single overlay layer drawn over the camera frame.
+
+    Each layer receives the active QPainter and the video rect (the letterboxed
+    area where the camera image was drawn) so it can position elements relative
+    to the actual video content if needed.
+
+    To add a new overlay (e.g. YOLO bboxes):
+        1. Subclass IOverlayLayer.
+        2. Implement paint().
+        3. Register with CameraView.add_overlay_layer().
+
+    No Qt subclassing is required — layers are plain Python objects.
+    """
+
+    @property
+    def name(self) -> str:
+        """Human-readable identifier used in menus / debug output."""
+        return type(self).__name__
+
+    @property
+    def visible(self) -> bool:
+        """Whether this layer should be drawn."""
+        return self._visible
+
+    @visible.setter
+    def visible(self, value: bool) -> None:
+        self._visible = value
+
+    def __init__(self) -> None:
+        self._visible: bool = True
+
+    @abstractmethod
+    def paint(self, painter: QPainter, video_rect: QRect) -> None:
+        """
+        Draw this layer.
+
+        Args:
+            painter:    Active QPainter on the CameraView widget.
+                        Caller saves/restores painter state around each layer.
+            video_rect: The QRect where the camera image was drawn (letterboxed).
+                        Use this to position overlays relative to the video image.
+        """