feat(csharp-scope): parity Unit 4 — callsite arity synthesis (infra)

Synthesize @reference.arity on every invocation_expression and
object_creation_expression by counting `argument` named children of
the backing `argument_list`. Wires the capture-to-Callsite pipeline
shared extractor already consumes (`scope-extractor.ts:878`).

No parity-count movement: the remaining arity-adjacent failures
(overload disambiguation, optional-parameter dedup, variadic
resolution) need type-based argument inference or member-call dedup,
both explicitly deferred in the plan's Known Limitations section.
This commit is infrastructure — future work lands on top of it.

Legacy 175/175 green; 21 parity failures remain.

gitnexus/src/core/ingestion/languages/csharp/captures.ts

@@ -143,6 +143,30 @@ export function emitCsharpScopeCaptures(
       }
     }
 
+    // Synthesize `@reference.arity` on every callsite so the
+    // registry's arity filter can narrow overloads. Count the
+    // `argument` named children of the backing `argument_list`.
+    // Python doesn't synthesize this today; C# needs it because the
+    // language has method overloading and the suite asserts overload
+    // resolution.
+    const callTag = (
+      ['@reference.call.free', '@reference.call.member', '@reference.call.constructor'] as const
+    ).find((t) => grouped[t] !== undefined);
+    if (callTag !== undefined && grouped['@reference.arity'] === undefined) {
+      const anchor = grouped[callTag]!;
+      const callNode =
+        findNodeAtRange(tree.rootNode, anchor.range, 'invocation_expression') ??
+        findNodeAtRange(tree.rootNode, anchor.range, 'object_creation_expression');
+      if (callNode !== null) {
+        const argList = callNode.childForFieldName('arguments');
+        const n =
+          argList === null
+            ? 0
+            : argList.namedChildren.filter((c) => c !== null && c.type === 'argument').length;
+        grouped['@reference.arity'] = syntheticCapture('@reference.arity', callNode, String(n));
+      }
+    }
+
     out.push(grouped);
 
     // Synthesize primary-constructor declarations on class/record