feat(graph): add Go language extractor

[0xmohit]

Add code-graph extraction support for Go, wired through the contributor
seam in docs/extractors.md: a GoWalker over the tree-sitter-go grammar,
registered in the extractor registry and the grammar map.

• Extract structs (class), interfaces, type aliases, functions,
  methods, generics, imports, calls, struct instantiations, and struct
  field types, with signatures, docstrings, visibility, and export state.
• Methods bind to their receiver type through a deferred second pass, so
  a method resolves to a type declared later in the file with a single
  containment parent and a receiver-stable identity.
• Vendor the grammar's own published wasm, tree-sitter-go@0.25.0
  (https://github.com/tree-sitter/tree-sitter-go), at
  src/graph/wasm/tree-sitter-go.wasm.
• Add fixture sample.go and 12 tests covering declarations, containment,
  references, generics, docstrings, multi-name declarations, and
  declaration-order independence.
• Retarget the coverage-histogram fixtures to .java now that .go is an
  indexed source extension, so unindexed-source examples stay accurate.
• Document Go support in docs/code-graph-support.md and the grammar
  provenance in docs/extractors.md.

Fixes #93

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What

Add code-graph extraction support for Go. A new GoWalker (in src/graph/extraction/languages/go.ts) parses .go files with the tree-sitter-go@0.25.0 grammar and emits:

• file, class (structs), interface, type_alias, function, method, property, variable, and constant nodes
• contains, imports, calls, instantiates, and type_of edges
• Signatures, docstrings, visibility/export state, return types, and type parameters

Methods bind to their receiver type via a deferred second pass, so a method resolves to a type declared later in the same file with a single containment parent and a receiver-stable identity.

The extractor is registered in src/graph/extraction/languages/index.ts and src/graph/extraction/grammars.ts (.go extension), and the grammar's own published tree-sitter-go.wasm (v0.25.0) is vendored under src/graph/wasm/.

Why

Closes #93. Go is one of the highest-demand languages for the code-graph; this is the first extractor for it, following the contributor workflow in docs/extractors.md.

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/Tooling

How to test

1. npm run typecheck
2. npm test (full suite, 4188 passing)
3. npm run build
4. Focused tests: npx vitest run src/graph/__tests__/extractor-go.test.ts — 12 tests covering structs, interfaces, type aliases, functions, methods, generics, imports, calls, struct field types, docstrings, multi-name declarations, and declaration-order independence.
5. Real-project run: mex graph rebuild on a 526-file Go repository (not public code) indexed 5826 files (5826/5826 parse ok, 0 failed), creating 262,054 nodes / 297,812 edges; mex graph query where-defined resolves project-internal functions exactly, and mex graph scope returns source-backed symbols with resolved calls/instantiates flows at confidence 1.

Checklist

• Tests pass (npm test)
• No breaking changes (or documented below)
• Tested locally with a real project

Code-graph changes

• This PR targets main
• A linked issue agrees on the bounded extractor/resolver scope
• The change follows the frozen LanguageExtractor or FrameworkResolver interface
• A focused fixture and assertions for the expected node/edge shape are included
• Any new grammar WASM, extension mapping, extractor, or resolver is registered
• No graph identity, reconciliation, schema, or drift-semantics changes are included, or a core / discuss-first issue is linked above