feat(python-scope): capture function return-type annotations

Unit 3 of the python migration architectural plan
(docs/plans/2026-04-19-001-refactor-python-migration-architectural-plan.md).

Wires the `def get_user() -> User` return-type annotation into the
typeBindings stream so the existing constructor-inferred + transitive
chain machinery can resolve `u = get_user(); u.save()` to `User#save`
without any orchestrator change.

Changes:
- `query.ts` + `scopes.scm`: new `@type-binding.return` pattern keyed by
  the function name (matches RFC §5.1 canonical vocabulary).
- `interpret.ts`: maps `@type-binding.return` to the existing
  `'return-annotation'` source label (no shared change needed).
- `scope-extractor.ts pass4CollectTypeBindings`: extends the Pass 2
  auto-hoist (anchor range == innermost scope range → bind in parent)
  to type bindings as well — return-type bindings whose anchor IS the
  function_definition land in the function's enclosing scope so
  callers see them.

Same-file return-type inference is now end-to-end:
  `def get_user() -> User: ...` + `u = get_user()` produces
  `u: User (return-annotation)` in the caller's scope via
  `followChainedRef`.

Verification:
- Flag-off: 191/191 (identical baseline).
- Flag-on: 31 fail / 160 pass (no change — every remaining
  return-type test in this fixture set is *cross-file*; carrying
  `get_user → User` across module boundaries lands with the
  cross-file typeBinding propagation work in Unit 5/7).
- tsc --noEmit clean.

gitnexus/src/core/ingestion/languages/python/interpret.ts

@@ -122,6 +122,7 @@ export function interpretPythonTypeBinding(captures: CaptureMatch): ParsedTypeBi
   else if (captures['@type-binding.constructor'] !== undefined) source = 'constructor-inferred';
   else if (captures['@type-binding.annotation'] !== undefined) source = 'annotation';
   else if (captures['@type-binding.alias'] !== undefined) source = 'assignment-inferred';
+  else if (captures['@type-binding.return'] !== undefined) source = 'return-annotation';
 
   return { boundName: nameCap.text, rawTypeName: rawType, source };
 }

gitnexus/src/core/ingestion/languages/python/query.ts

@@ -113,6 +113,16 @@ export const PYTHON_SCOPE_QUERY = `
   left: (identifier) @type-binding.name
   type: (type) @type-binding.type) @type-binding.annotation
 
+;; Return-type annotation: \`def get_user() -> User:\` binds the
+;; FUNCTION'S NAME to its return type in the enclosing scope. Combined
+;; with the constructor-inferred + chain-follow path, \`u = get_user()\`
+;; then resolves \`u: User\` cross-call. The Python provider hoists the
+;; binding via \`pythonBindingScopeFor\` to the function's parent scope
+;; so callers in module/class scope see it.
+(function_definition
+  name: (identifier) @type-binding.name
+  return_type: (type) @type-binding.type) @type-binding.return
+
 ;; References — calls
 (call
   function: (identifier) @reference.name) @reference.call.free

gitnexus/src/core/ingestion/languages/python/scopes.scm

@@ -125,6 +125,18 @@
   left: (identifier) @type-binding.name
   type: (type) @type-binding.type) @type-binding.annotation
 
+; ─── Type bindings: function return-type annotations ─────────────────────
+;
+; `def get_user() -> User:` — binds the function's NAME to its return
+; type in the enclosing scope. Combined with the constructor-inferred +
+; chain-follow path, `u = get_user()` then resolves `u: User` cross-
+; call. Python provider hoists the binding via `pythonBindingScopeFor`
+; to the function's parent scope so callers in module/class scope see it.
+
+(function_definition
+  name: (identifier) @type-binding.name
+  return_type: (type) @type-binding.type) @type-binding.return
+
 ; ─── References: calls ─────────────────────────────────────────────────────
 ;
 ; Free call:   `print(x)`     — function is a bare identifier

gitnexus/src/core/ingestion/scope-extractor.ts

@@ -666,9 +666,19 @@ function pass4CollectTypeBindings(
     const innermost = draftById.get(innermostId);
     if (innermost === undefined) continue;
 
+    // Auto-hoist for scope-creating type bindings (e.g. Python's
+    // `@type-binding.return` whose anchor is the function_definition
+    // itself). Same condition as Pass 2 — when the anchor coincides
+    // with the innermost scope's range, the binding belongs in the
+    // enclosing scope (callers, not the function body, look up the
+    // return type by the function's name).
+    const autoHostedId =
+      innermost.parent !== null && rangesEqual(anchor.range, innermost.range)
+        ? innermost.parent
+        : innermost.id;
     // `bindingScopeFor` may hoist the type binding to an outer scope.
     const hostId =
-      provider.bindingScopeFor?.(match, draftToScope(innermost), scopeTree) ?? innermost.id;
+      provider.bindingScopeFor?.(match, draftToScope(innermost), scopeTree) ?? autoHostedId;
     const host = draftById.get(hostId) ?? innermost;
 
     const typeRef: TypeRef = {