feat(graph): add Python code graph extractor

[abhinav-phi]

What

• Add a Python LanguageExtractor for the code graph.
• Register Python extraction and .py file detection.
• Vendor and register the Tree-sitter Python WASM grammar.
• Add representative Python fixtures and focused extractor tests.
• Emit file, class, function, method, module variable, constant, import, call, inheritance, instantiation, and containment graph information while preserving unresolved cross-file references via targetName.

Why

Closes #94.

This adds first-class Python language support to the code-graph preview branch while preserving the frozen LanguageExtractor interface and existing graph semantics.

Type of change

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

How to test

1. Run:

   npm run typecheck

2. Run:

   npm test

3. Run:

   npm run build

4. Verify:
   • Python files (.py) are detected correctly.
   • Python extractor emits file, class, function, method, import, call, inheritance, instantiation, and containment nodes/edges.
   • Cross-file references remain unresolved through targetName.
   • The Python grammar WASM is bundled into the build output.

Checklist

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

Code-graph preview

• This PR targets code-graph-preview, not main
• The linked issue states Target branch: code-graph-preview
• 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