fix(csharp): resolve chained static-factory calls Foo.Create().Bar() (#750) (#753)

A C# method called through a static factory or fluent chain —
`Foo.Create().Bar()`, `JObject.Parse(s).Property(...)`,
`Instant.FromUtc(...).InZone(zone)` — lost the receiver's type, so the chained
method didn't resolve and the call was invisible to callers/impact/trace. Ports
the #645/#608 mechanism to C# (additive, like Java #751):

- Part 1: capture C# return types in the extractor, reading the `returns` field
  (`static Foo Create()` -> `Foo`); predefined/array/generic/nullable/namespaced
  types are normalized or skipped.
- Part 2: encode a chained `member_access_expression` receiver
  (`Foo.Create(args).Bar()`) as `inner().Bar` with normalized empty parens, so
  factory calls that take arguments still split. Non-chained member calls keep
  their existing `recv.Method` text.
- Part 3: resolve via the shared matchDottedCallChain (now Java/Kotlin/C#),
  validated by resolveMethodOnType so a wrong inference yields NO edge.

Known limitation (safe): C# extension-method chains don't resolve, since the
method lives on the extension class, not the receiver's type — no edge, never a
wrong one.

Validated: synthetic decoy + args + absent-method safety tests; full suite green;
real-repo A/B on Newtonsoft.Json (945 .cs: +3, 0 lost) and nodatime (488 .cs:
+73, 0 lost) — node count identical (no explosion), 0 edges lost, precision
spot-checked verbatim (Instant.FromUtc().InZone(), Offset.FromHoursAndMinutes().Plus(),
OffsetDateTimePattern.CreateWithInvariantCulture().WithTwoDigitYearMax()).
EXTRACTION_VERSION 7 -> 8.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/extraction/languages/csharp.ts

@@ -32,6 +32,26 @@ export function blankCsharpPreprocessorDirectives(source: string): string {
   return source.replace(re, (m, indent) => indent + ' '.repeat(m.length - indent.length));
 }
 
+/**
+ * A C# method's declared return type, normalized to the bare class name a chained
+ * `Foo.Create().Bar()` could be called on (the #645/#608 mechanism). The return
+ * type lives in the `returns` field (`static Foo Create()` → `Foo`); built-in
+ * `predefined_type` (void/int/string/…) and arrays yield undefined, generics are
+ * unwrapped to the base type, nullable `Foo?` is stripped, and a dotted namespace
+ * is reduced to the simple name. Constructors have no `returns` field → undefined.
+ */
+function extractCsharpReturnType(node: SyntaxNode, source: string): string | undefined {
+  const typeNode = node.childForFieldName('returns');
+  if (!typeNode) return undefined;
+  if (typeNode.type === 'predefined_type' || typeNode.type === 'array_type') return undefined;
+  let t = getNodeText(typeNode, source).trim();
+  t = t.replace(/\?+$/, ''); // nullable `Foo?`
+  t = t.replace(/<[^>]*>/g, ''); // generics `List<Foo>` → `List`
+  const last = t.split('.').pop()?.trim(); // namespace `Ns.Foo` → `Foo`
+  if (!last || !/^[A-Za-z_]\w*$/.test(last)) return undefined;
+  return last;
+}
+
 export const csharpExtractor: LanguageExtractor = {
   preParse: blankCsharpPreprocessorDirectives,
   functionTypes: [],
@@ -67,6 +87,7 @@ export const csharpExtractor: LanguageExtractor = {
   bodyField: 'body',
   paramsField: 'parameters',
   returnField: 'type',
+  getReturnType: extractCsharpReturnType,
   getVisibility: (node) => {
     for (let i = 0; i < node.childCount; i++) {
       const child = node.child(i);