feat(allowlist): add Shader Languages support

Sub-issue of #470.

Add support for shader source files to the review allowlist:
- Extensions: `.glsl`, `.hlsl`, `.wgsl`, `.metal`
- No conventional test-file exclude pattern (shader source has no widely-adopted test-directory convention across GLSL/HLSL/WGSL/Metal toolchains)
- A shared `shader.md` review rule doc covering precision/NaN handling, texture and buffer bounds safety, cross-stage binding/layout contracts, compute-shader synchronization, and GPU-specific performance/security pitfalls
- Registration in `system_rules.json`

Will follow with a PR.

Start by locating system_rules.json and the existing language review-rule entries that the new shader registration should follow. Add the shared shader.md rule covering the listed safety and performance concerns, register the four extensions, and verify that no test-file exclusion pattern is added and all entries are recognized by the allowlist.

Written by the indexing model from the issue text.