feat(rig): add CameraRig.grid planar camera-wall preset

[dchaudhari7177]

What

CameraRig had ring, stereo, arc, line, custom, and stations presets but no planar grid — a common capture setup (camera wall / light-stage front).

Adds CameraRig.grid(rows, cols, corner, right, down, look_at, *, width, height_px, focal|fov_deg, overrides=None): camera (r, c) sits at corner + c*right + r*down, returned row-major with id r*cols + c, all facing look_at. Built through Camera.look_at (like the other presets), so the RDF / Z-up / t = -R @ C convention is never re-derived. Modeled on CameraRig.line and shares its _intrinsics / _resolve_overrides plumbing.

Acceptance criteria

• ✅ returns rows*cols cameras spanning the plane defined by corner + right/down spacing vectors
• ✅ reuses _intrinsics(...) (exactly one of focal/fov_deg) and the existing overrides shape (sequence or index-keyed mapping)
• ✅ validates rows >= 1, cols >= 1
• ✅ a test asserts count, that every camera faces look_at (project(look_at) lands on the principal point), and t = -R@C holds (centre() round-trips to the grid position)
• ✅ mypy, ruff check, ruff format --check, pytest green

Tests

tests/test_rig_grid.py (standalone — imports only CameraRig, no triangulation-reader dependency): the 2×3 facing/convention test above, an fov + overrides case, non-positive rows/cols rejection, and the exactly-one-of focal/fov guard. 4 passed.

Fixes #51