Shader feasibility probe

See the handoff for pins, commands, independent binding/layout contracts, evidence, and hardware gaps.

probe.slang is the only shader source. scripts/shader-probe bootstrap acquires verified host tools; compile is offline and writes generated artifacts under out/. Enable LUDUS_BUILD_SHADER_PROBE for the native executable and CTests. The browser harness consumes generated WGSL through real WebGPU. The test runner uses the existing Playwright lock. Neither target is installed into Ludus's SDK.