feat(extraction): index Metal shader files (.metal) via the C++ grammar (#1121) (#1151)

.metal was absent from EXTENSION_MAP, so Metal Shading Language files were
silently skipped. MSL ≈ C++14, and the C++ grammar extracts its functions,
structs, type aliases, and call edges at parity with plain C++ — except MSL's
post-declarator [[attribute]] annotations, which misparse struct fields into
spurious extends refs from the struct to the field's own type (a wrong
inheritance edge whenever the repo typedefs float3/float4x4 itself, common in
shared ShaderTypes.h). blankMetalAttributes blanks them pre-parse,
offset-preserving, following the blankCppExportMacros pattern (#1061), gated
to .metal files only — in regular C++ the attribute position is legal syntax
the grammar parses natively. The preParse hook gains an optional filePath
param to support the gate.

Validated on llama.cpp's ggml-metal.metal (10.7k lines: 130 kernels vs 113
`kernel void` ground-truth lines, rope_yarn resolves its 4 kernel callers)
and SDL's shaders (PQtoLinear ← GetOutputColor), 0 bogus extends edges.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/extraction/grammars.ts

@@ -108,6 +108,10 @@ export const EXTENSION_MAP: Record<string, Language> = {
   '.luau': 'luau',
   '.m': 'objc',
   '.mm': 'objc',
+  // Metal Shading Language ≈ C++14: the C++ grammar extracts its functions,
+  // structs, and calls. MSL-specific `[[attribute]]` annotations are blanked
+  // pre-parse for `.metal` files (see blankMetalAttributes in c-cpp.ts). (#1121)
+  '.metal': 'cpp',
   // XML: file-level tracking; the MyBatis extractor matches `<mapper namespace="...">`
   // shape and emits SQL-statement nodes (other XML returns empty).
   '.xml': 'xml',