[crertel]

Adds camera and billboard transforms to Mat44:

• orient(position, forward, up) maps local coordinates into world space.
• look_at(eye, center, up) maps world coordinates into camera space and is the inverse of the corresponding orientation transform.
• make_billboard(position, camera_position, camera_up) creates a spherical billboard.
• make_billboard_axis(position, camera_position, axis) creates a cylindrical billboard whose local +Y stays aligned with the supplied axis.

All four use right-handed row-vector transforms, with local -Z forward/front, +Y up, and +X right. The view matrix sends the eye to the origin and its target onto negative Z, following the OpenGL viewing convention. Direction inputs are normalized internally, with scaling to handle very large or small magnitudes. Zero directions, coincident positions, and parallel or nearly parallel directions raise ArithmeticError; the angular degeneracy threshold is documented as a normalized cross-product length of 1.0e-12, consistent with the existing orthonormalization tolerance.

Float typespecs and explicit float clauses are preserved, with numeric fallback clauses. The docs distinguish local-to-world transforms from view matrices and specify the billboard front direction.

Validation: 579 tests and 5 doctests pass with 100% line coverage. New tests cover known axis-aligned and oblique bases, view/pose inverses, all signed coordinate axes, spherical versus cylindrical behavior, oblique rotation axes, translation and homogeneous coordinates, mixed numeric inputs, magnitude scaling, and degenerate inputs. Formatting, compilation with warnings as errors, and documentation generation pass.

Part of #44. Projection helpers will follow separately.

[coveralls]

coverage: 100.0%. remained the same — 44-mat44-camera into master