Rendering — adopt `ViewBasis` for host primary rays

What

Wire `Novolis.Math.Geometry.ViewBasis` into the Rendering host path so primary-ray direction math lives in one tested place:

ViewBasis and RigidTransform exist in Geometry but no consumer references them yet (grep 2026-05-25).

Why

• Dogfood and trace apps already build observers via `ViewPose` (Simulation) or `CameraSnapshot.LookAt` (Rendering); the basis math is duplicated twice with slightly different epsilon handling.
• Centralizing in Math keeps Simulation free of Rendering while giving Rendering a single dependency for ray generation (allowed: Rendering → Math).
• Reduces risk that CPU path tracer and future software rasterizers diverge from Simulation pick rays (BuildPickRay in RagdollPlay, etc.).

How

1. `CameraSnapshot` (Novolis.Rendering.Runtime)

• In LookAt, call ViewBasis.FromLookAt(position, target, up) and copy Forward, Right, Up into the record.
• Keep public surface unchanged (no new package dep beyond existing Geometry).

1. `PathTracerEngine` (Novolis.Rendering.Backends.Cpu)

• Construct var basis = new ViewBasis(camera.Forward, camera.Right, camera.Up) once per tile or frame (or add CameraSnapshot.ToViewBasis() helper on Rendering side using those fields).
• Replace lines 52–54 with ViewBasis.PrimaryRayDirection(in basis, u, v, tanHalfFov, aspect).

1. Tests

• Unit test: CameraSnapshot.LookAt vs ViewBasis.FromLookAt — forward/right/up agree within GeometryConstants scale.
• Golden pixel or direction test: one pixel center ray matches previous implementation (regression).

1. Optional follow-up (apps, not platform)

• RagdollPlay BuildPickRay can mirror PrimaryRayDirection for mouse picking consistency.

Out of scope

• Moving GGX/glass/sky shading into Math (Rendering-only).
• ILGPU device kernels (see ilgpu-bvh-slab-parity.md).

Acceptance

• No manual Cross/Normalize basis construction in PathTracerEngine for primary rays.
• CameraSnapshot and ViewBasis stay aligned under test.