feat: extract Metal (.metal) kernels into the graph

[GoodOlClint]

What

Adds Metal Shading Language (.metal) support to graphify by routing it through
the existing C++ extractor — the exact reuse pattern CUDA's .cu/.cuh
already follow. MSL is C++14, so extract_cpp (tree-sitter-cpp) handles it with
no new grammar. ERROR nodes around Metal-specific qualifiers (kernel,
[[buffer(0)]]) are expected and harmless, just as they are for CUDA's
__global__/__device__.

Why

Before this change graphify classified .metal as non-code and skipped it
entirely, so MLX/Metal compute kernels never appeared in the graph. This lets
them join it like any other source.

Approach

Mirror CUDA. No dedicated Metal grammar — deferred. ~3-line change across the
same three touchpoints CUDA uses.

Edits (3 touchpoints)

1. graphify/detect.py — '.metal' added to CODE_EXTENSIONS (next to .cu/.cuh).
2. graphify/extract.py — ".metal": extract_cpp in the extension→extractor dict.
3. graphify/build.py — ".metal": "cpp" in _LANG_FAMILY, so Metal↔C++ count
   as one family for cross-language seam / import resolution.

Plus a contract test + tests/fixtures/sample.metal (a real MSL snippet: a
struct + a dot3 function + a saxpy kernel).

Evidence (before → after)

Contract test (tests/test_languages.py, mirrors the cpp/CUDA tests):
extract_cpp(FIXTURES / "sample.metal") — error absent, Vec3 struct and
dot3 function both appear in nodes. Symbols graphify produced nothing for before.

E2e — GRAPHIFY_OUT=<tmp> graphify update <dir-with-.metal> on a dir
containing one kernel.metal:

	graph.json	metal-source nodes
before (origin/v8)	not created — No code files found - nothing to rebuild	0 (skipped)
after (this branch)	Rebuilt: 11 nodes, 10 edges, 4 communities	7

After-state metal nodes: kernel.metal, Vec3, x, y, z, dot3(), saxpy().

Test gate — uv run pytest (full suite): 2351 passed, 28 skipped
(includes 3 new test_metal_*). ruff check on changed files: clean.

Notes

• cpp-reuse, not a new extractor — CUDA precedent; extract_cpp is the point.
• Dedicated Metal grammar deferred.
• Branched off v8; pushed to fork GoodOlClint. PR not opened.

[safishamsi]

Thanks @GoodOlClint, and apologies for the overlap. Metal support has landed on v8 (0b33511) — same .metal→C++ approach and fixture as yours, with the additional _LANG_FAMILY map fix and README update from #1480. Your PR had drifted behind v8 (detect.py conflict). Closing as superseded, with credit to you noted in the commit for the original approach. Thank you for the contribution.