[Python] Add code-graph extractor

[theDakshJaitly]

Target branch: code-graph-preview

Goal

Add the first community language extractor by implementing Python support against the frozen LanguageExtractor interface. The first version should provide a reliable structural graph for common Python modules without attempting framework-specific behavior.

Scope

• Detect .py files as python.
• Vendor and register a compatible Tree-sitter Python WASM grammar.
• Emit a file node and containment tree.
• Extract functions, async functions, classes, methods, module variables, and constants where they can be identified reliably.
• Capture signatures, documentation strings, export visibility conventions, and return annotations when present.
• Emit imports, calls, containment, inheritance, and instantiation references.
• Leave cross-file targets unresolved through targetName for the existing resolution pass.

Required changes

• Add src/graph/extraction/languages/python.ts.
• Register the extractor in src/graph/extraction/languages/index.ts.
• Register .py and the grammar filename in src/graph/extraction/grammars.ts.
• Add the grammar WASM under src/graph/wasm/ and document its source, version, and license.
• Add a representative Python fixture under src/graph/__tests__/fixtures/.
• Add focused extractor tests under src/graph/__tests__/.

Acceptance criteria

• The extractor implements the frozen LanguageExtractor interface without changing it.
• Extraction is pure, deterministic, and limited to one file.
• Node ids use generateNodeId(filePath, kind, name).
• Tests cover file identity, functions, classes, methods, qualified names, containment, imports, calls, inheritance, and instantiation.
• Cross-file references remain unresolved for the engine to bind.
• Unsupported or malformed syntax degrades without crashing graph construction.
• npm run typecheck, npm test, and npm run build pass.
• The packed build contains the Python grammar WASM.

Out of scope

• Django, Flask, FastAPI, or other framework semantics.
• Runtime import execution or Python environment inspection.
• Type inference beyond explicit syntax.
• Changes to identity, reconciliation, schema, or grounding semantics.

Contributor references

• Extending the code graph
• src/graph/extraction/languages/typescript.ts
• src/graph/__tests__/extractor.test.ts

Contributor workflow

git switch code-graph-preview
git pull
git switch -c feat/python-code-graph-extractor

Open the pull request with base branch code-graph-preview.