feat: mesh materials and mesh shadows - #429
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renderer.py becomes paz/graphics/renderer/, one module per stage of the pipeline: rays (the bounce loop), tiling, intersect, shade, shadow, optics, material. The loop body now reads intersect -> shade -> advance instead of burying the scatter step inside update_state. Avoids "trace", which in a JAX-first library already means jit tracing: trace_bounces is rays.render, trace_chunks is rays.render_chunks. The four shape-rendering examples patched paz.graphics.renderer.compute_soft_occlusion, which the split would have silently turned into a no-op; they now patch renderer.shadow directly. The tiling module is not called "tiles" because render() takes a public tiles= keyword that examples pass by name. No behaviour change. 106 tests pass in renderer_test.py and mesh_test.py including all snapshots; all 16 benchmark gradients are bit-identical to the pre-split values.
scene.stack_materials already compiles reflective, transparency and refractive_index for every mesh, and both Material and CookTorranceMaterial declare all three. The renderer threw them away and substituted a hardcoded (0.0, 0.0, 1.0) row for all triangles, so a mirror or glass mesh rendered as matte and opaque. Gather the real values per primitive via triangle_hit.primitive and select between shape and triangle values on the closest hit. Meshes that leave these fields at their defaults render identically, so existing scenes are unaffected. Two new mesh_test cases cover a reflective and a transparent mesh; both fail before this change.
Shadow rays only ever intersected shapes, so a mesh lit from above cast nothing onto the floor beneath it. Shadow rays now also traverse the triangles and join the shape blockers as one more caster row before the soft-occlusion reduction. select_meshes never received shadow_mask, so the public "this node does not cast" control had no mesh equivalent. CompiledScene gains triangle_shadow_mask and scene.compile fills it, which keeps that control working for meshes instead of silently ignoring it. The shape path rejects a shadow ray re-hitting its own surface by shape identity. Triangles have no such index, so they use a distance threshold instead; meshes do not receive shadows yet, so today the receiver is always a shape and the threshold only guards shape receivers close to a mesh. Two mesh_test cases cover casting and the mask suppressing it; both fail before this change.
The invented TRIANGLE_SELF_HIT_EPSILON was unexercised: sweeping it over five orders of magnitude across three scene layouts produced byte-identical renders, because with only shape receivers a shadow ray can reach a triangle at near-zero depth only if the shape is coincident with the mesh. Use the EPSILON the shape path already applies to non-self hits instead of a new magic number.
compute_triangle_colors shaded with compute_colors and no occlusion, so a mesh was never darkened by anything and a mesh-only scene had no shadows at all regardless of the shadows flag. Triangles now take the same per-light occlusion the shape path uses. compute_occlusion takes a Receiver (points, normals, shape indices) instead of a Hit, so triangle hit points can be fed in; a triangle receiver uses NO_SHAPE so the shape self-hit rejection never matches it. Shape blockers are now skipped when a scene has no shapes, which a mesh-only scene with shadows would previously have crashed on. Reinstates TRIANGLE_SELF_HIT_EPSILON, this time with evidence: on a sphere mesh self-shadowing at its terminator, values at or below 1e-5 leak self-hits, the effect saturates at 1e-3, and 1e-3 to 5e-2 are flat. The spurious self-shadow it removes is worth up to 9 grey levels.
feat: mesh materials and mesh shadows
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#425 merged into refactor/renderer-style rather than paz-jax, because that was
its base as a stacked PR. The five mesh commits are therefore sitting on
refactor/renderer-style and never reached paz-jax. This lands them.
Contents are exactly what #425 reviewed: the renderer package split, plus
meshes honouring reflective/transparency/refractive_index, casting shadows and
receiving them. No new commits since that review.
Merge base is 481f4f3 and the branch diff is only these changes, so it applies
cleanly; a fast-forward is impossible only because #424's merge commit was
created on paz-jax.