fix(extraction): C# produces references edges for type annotations (#381) (#470)

Indexing any C# project produced zero `references` edges, so
`codegraph_callers SomeDto` returned no hits even when the DTO was used
as a param/return type across the codebase, and `codegraph_callees` on
a service class only saw its `using` imports — the headline structural
query silently degraded to text-search on half of every typical backend
stack.

Two root causes:

1. `csharp.ts` was missing `returnField` (default `'return_type'` doesn't
   exist on C# AST; the field is `'type'`) AND had
   `paramsField:'parameter_list'` (the node TYPE, not the field NAME
   `'parameters'`) — so parameter type extraction silently no-op'd.
2. `extractTypeRefsFromSubtree` only emitted refs for `type_identifier`
   leaves. C# tree-sitter doesn't produce `type_identifier` — it uses
   `identifier`, `predefined_type`, `qualified_name`, `generic_name`,
   `array_type`, `nullable_type`, `tuple_type`, etc.

Fix:

- `csharp.ts`: `paramsField:'parameters'`, `returnField:'type'`.
- Route C# through a dedicated `extractCsharpTypeRefs` +
  `walkCsharpTypePosition`. Descends ONLY into known type fields
  (`parameter.type`, `method.type`, `property.type`,
  `variable_declaration.type`, `tuple_element.type`), so parameter
  NAMES like `request` in `Build(UserDto request)` never leak as type
  refs.
- Hook `extractField` and `extractProperty` to call
  `extractTypeAnnotations` so property/field type refs land in the graph.

Validation on dotnet/eShop (527 .cs files):
  C# `references` edges: 35 -> 925 (+26x)
  No regression in calls/imports/instantiates/extends/implements.

Closes #381.

src/extraction/languages/csharp.ts

@@ -18,7 +18,8 @@ export const csharpExtractor: LanguageExtractor = {
   propertyTypes: ['property_declaration'],
   nameField: 'name',
   bodyField: 'body',
-  paramsField: 'parameter_list',
+  paramsField: 'parameters',
+  returnField: 'type',
   getVisibility: (node) => {
     for (let i = 0; i < node.childCount; i++) {
       const child = node.child(i);