Add eForth raytracer showcase

[jserv]

tests/raytracer.fth runs on the baked 32-bit eForth and raytraces two spheres over a checkerboard with shadow rays and half-Lambert + Phong shading, streaming one deterministic 64x32 RGB888 PNG to stdout via the svpng stored-block technique (quadratic ray-sphere intersection, a bitwise isqrt, CRC32 from a 16-entry nibble table, Adler32, one uncompressed DEFLATE block per scanline). The substrate is the unmodified build/muxleq driven by stdin: no VM, CLI, or MMIO change.

Arithmetic is integer-only 16.16 fixed point (rescaled x256 from nand2raytrace's 8.8), so the output is byte-stable across runs and machines. It is also portable across Forth implementations: the same source runs on gforth and emits the identical 6399-byte PNG. The CRC folds are masked to 32 bits (a no-op on muxleq's 32-bit cell) and the prompt/warning setup lines are guarded with defined ... [if] so gforth skips what it lacks.

The golden-raytracer target byte-compares the render against tests/expected/raytracer.png. It stays standalone rather than in the fast check gate, since one frame is a few minutes of pure-eForth interpretation.

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Summary by cubic

Adds a deterministic eForth raytracer showcase and a golden-raytracer Makefile target that verifies the PNG output byte‑for‑byte. It renders a 64x32 RGB888 frame to stdout using the unmodified build/muxleq interpreter.

• New Features
  • New test tests/raytracer.fth: 16.16 fixed‑point raytracer (two spheres, checkerboard, shadows, half‑Lambert + Phong).
  • Portable and byte‑stable: identical PNG on build/muxleq and gforth.
  • Makefile: adds golden-raytracer (respects RAYTRACE_TIMEOUT, default 600s; warns if no timeout) that byte‑compares against tests/expected/raytracer.png; excluded from check due to multi‑minute runtime.
  • No VM/CLI/MMIO changes.

^{Written for commit 0227c87. Summary will update on new commits.}