Add a Camera.forward() getter (world-space viewing direction)

[bamdadd]

Camera.centre() (src/multicam_sim/cameras.py:151) reads the camera position off R/t, but there's no getter for the direction the camera is looking — handy for logging, frustum checks, and "is this camera pointed the right way?" sanity tests.

Add Camera.forward() -> FloatArray: the world-space unit viewing direction.

Pointer: look_at_rotation (src/multicam_sim/geometry.py:32) stores the camera axes as the rows of R in the order [right, down, forward] (OpenCV convention, +z forward), so the forward axis is self.rotation()[2]. Put the getter next to centre() (cameras.py:151), returning a length-3 array.

Acceptance criteria

• forward() returns a length-3 world-space unit vector (np.linalg.norm ≈ 1).
• Round-trip test: a camera from Camera.look_at(0, intr, eye, target) has forward() ≈ normalize(target - eye).
• Pure read-only; no new dependency.
• uv run mypy src, ruff check, ruff format --check, pytest green.