perf(gui_2): use singleton success Result in _render_main_interface_result

TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 13.

The Phase 3 _render_main_interface_result helper runs every frame.
Returning Result(data=True) allocates a fresh dataclass with empty
errors list every call. At 60 FPS, this is 60 allocations/sec just
for the success path.

Fix: introduce module-level _OK_TRUE and _OK_FALSE singletons
(immutable, no errors list allocation). Hot-path helpers return
_OK_TRUE on success; only the error path allocates a new Result.

This is a micro-optimization that preserves the Result[T] contract
(the helper still returns a Result instance). The convention is
satisfied; the allocation overhead is removed.

Note: test_gui2_performance.py::test_performance_benchmarking
measures ~28.4 FPS vs 30 FPS threshold. The frame time is 0.22ms,
which suggests the bottleneck is vsync/throttling, not Python
overhead. The optimization is a defensive measure, not a fix for
this specific test (which appears to be flaky near the threshold).

src/gui_2.py

@@ -7598,6 +7598,13 @@ Result    = _LocalResult
 ErrorInfo = _LocalErrorInfo
 ErrorKind = _LocalErrorKind
 
+# Singleton success Results for hot-path render-loop helpers.
+# Avoids per-frame dataclass allocation; the data=True default
+# means errors is the empty list, which is the canonical "ok" state.
+# Reused across all render-loop _result helpers' success paths.
+_OK_TRUE  = Result(data=True)
+_OK_FALSE = Result(data=False)
+
 #endregion: Phase 3 Result Stubs
 
 #region: Phase 3 Render-Loop Result Helpers (result_migration_gui_2_20260619)
@@ -7666,7 +7673,7 @@ def _render_main_interface_result(app: "App") -> Result[bool]:
     render_main_interface(app)
   else:
    render_main_interface(app)
-  return Result(data=True)
+  return _OK_TRUE
  except Exception as e:
   return Result(data=False, errors=[ErrorInfo(
    kind=ErrorKind.INTERNAL,