Add MediaPipe Hand & Gesture Detector block (JdeRobot/VisualCircuit#470)

[prawnsgupta]

Implements the Hand & Gesture Detector block approved in JdeRobot/VisualCircuit#470 (thanks @BkPankaj for the go-ahead).

What it does: runs MediaPipe's Hand Landmarker on the input image, detects up to two hands (21 landmarks each), and estimates a simple gesture (extended-finger count). Fills the gap in VisualCircuit's CV blocks (FaceDetector/ObjectDetector exist, no hand/landmark block) and pairs with MotorDriver/Teleoperator for gesture-driven robotics.

• Input: BGR image (Img)
• Outputs (via OutputType): image (annotated frame with 21-point skeleton + gesture label) or landmarks (flat array of 21 (x,y) landmarks + finger count)
• Parameters: OutputType (image/landmarks), MirroredInput (yes/no for pre-mirrored feeds)

Implementation notes:

• MediaPipe Tasks API (mediapipe.tasks.vision.HandLandmarker) — the legacy mp.solutions.hands API was removed in 0.10.30+, so this is the durable choice.
• Finger counting is rotation-invariant (distances from wrist, not raw x/y), verified on tilted/rotated hands.
• Thumb uses a dedicated pinky-MCP reference (the thumb moves laterally, not radially), so a closed/tucked thumb is detected correctly.
• Verified on real hand photos and a real thumbs-up image.

Needs a maintainer decision:

• New runtime dependency mediapipe (declared in the block's dependencies).
• Bundled model hand_landmarker.task (~7.6 MB) — should it live under utils/models/hand_landmarker/ alongside the Haar/YOLO models?

Demo video below.

Hand.Gesture.Demo.Video.mp4