feat(allowlist): add Shader Languages support

[wakqasahmed]

Summary

• Add .glsl, .hlsl, .wgsl, .metal to the supported file types allowlist
• No default exclude pattern added — shader source files have no conventional test-directory layout shared across GLSL/HLSL/WGSL/Metal toolchains, so allowed_ext_test.go documents this explicitly (matching the existing Prisma precedent)
• Add internal/config/rules/rule_docs/shader.md, a shared shader review rule doc covering precision/NaN propagation, texture and buffer bounds safety (including non-uniform control flow around texture sampling and compute writes), cross-stage binding/layout contracts (std140/std430, register/space, @binding/@group), compute-shader synchronization (barriers, race conditions), GPU performance anti-patterns (dynamic branching, texture-dependent reads), and security/portability concerns (untrusted buffer indices, vendor-specific intrinsics without fallback)
• Register **/*.{glsl,hlsl,wgsl,metal} -> shader.md in system_rules.json
• Add test coverage: allowed-extension matching (all four extensions, case-insensitive), non-exclusion of shader files, and rule resolution

Part of the language-allowlist expansion tracked in #470.

Closes #974

Test plan

• go test ./internal/config/allowlist/... -run 'TestIsAllowedExt|TestIsExcludedPath' -v — PASS
• go test ./internal/config/rules/... -run TestResolve_DefaultRules -v — PASS
• gofmt -s -l . — clean
• go vet ./internal/config/... — clean
• go run scripts/verify-english-only.go — clean (423 files scanned, no unapproved non-English text)

[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 2 selected item(s).

[wu21-web]

Site documentation updates missing, update at pages/src/content/docs/<locale>/review-rules.md.

[wakqasahmed]

@wu21-web Thanks for the catch — docs added in the latest commit. Updated pages/src/content/docs/{en,ja,ru,zh}/review-rules.md with a row for the new **/*.{glsl,hlsl,wgsl,metal} → shader.md mapping, mirroring the format used for the Elm entry (#978).

Note: the ja/ru/zh translations are my best effort — please have a native speaker double-check the phrasing before merge.

[wu21-web]

@wakqasahmed I will review jp and zh for you, but please reslove the merge conflicts first.

[wakqasahmed]

@wu21-web Resolved — rebased onto latest main and merged, fixing the conflicts in supported_file_types.json, system_rules.json, the two allowlist/rules test files, and all four locale review-rules.md docs (they were conflicting with the recently-merged jsonnet/zig/thrift/capnp entries). Ready for the ja/zh review whenever you get a chance.

[wu21-web]

Approved. Why did you say "the ja/ru/zh translations are my best effort" 🤔 , they didn't need a review really. Good job though.