fix(tui): use correct DiffDisplayMode enum members + typed compose() result

The diff-mode cycle in PermissionsScreen referenced DiffDisplayMode.SIDE_BY_SIDE
and DiffDisplayMode.CONTEXT, but the enum only defines UNIFIED / SPLIT / AUTO.
Pyright flagged both as reportAttributeAccessIssue and behave failed to import
the screen module, masking the entire scenario suite under a single
traceback-outside-scenario error.

Also addresses the prior re-review feedback on the same PR:
- compose() return type was Any; tighten to collections.abc.Iterator[Any] so the
  generator shape is exposed to type checkers without taking a hard textual
  dependency at typecheck time (Iterator[Any] is the structural type of a
  Textual ComposeResult; we stay importable when textual is absent).
- The Bug #10488 TDD scenario asserting action methods now also covers
  action_dismiss_screen so a future refactor cannot silently drop the escape
  binding without test failure.

Verified locally on this worktree:
- typecheck gate: 0 errors, 4 unrelated warnings.
- unit_tests gate on features/tui_permissions_screen.feature: 65/65 scenarios
  pass.
- lint gate: clean.

ISSUES CLOSED: #10488

features/tui_permissions_screen.feature

@@ -396,3 +396,4 @@ Feature: TUI PermissionsScreen
     And PermissionsScreen should have action method "action_nav_next"
     And PermissionsScreen should have action method "action_nav_prev"
     And PermissionsScreen should have action method "action_cycle_diff"
+    And PermissionsScreen should have action method "action_dismiss_screen"

src/cleveragents/tui/permissions/screen.py

@@ -10,6 +10,7 @@ from __future__ import annotations
 
 import contextlib
 import importlib
+from collections.abc import Iterator
 from typing import Any, ClassVar
 
 from cleveragents.tui.permissions.models import (
@@ -45,8 +46,8 @@ _ScreenBase = _load_screen_base()
 
 _DIFF_MODE_CYCLE: list[DiffDisplayMode] = [
     DiffDisplayMode.UNIFIED,
-    DiffDisplayMode.SIDE_BY_SIDE,
-    DiffDisplayMode.CONTEXT,
+    DiffDisplayMode.SPLIT,
+    DiffDisplayMode.AUTO,
 ]
 
 
@@ -167,8 +168,14 @@ class PermissionsScreen(_ScreenBase):
         """
         self._text = text
 
-    def compose(self) -> Any:
-        """Compose the screen layout with a Static widget showing the content."""
+    def compose(self) -> Iterator[Any]:
+        """Compose the screen layout with a Static widget showing the content.
+
+        Returns a generator over Textual ``Widget`` instances (the
+        ``ComposeResult`` type), but typed as ``Iterator[Any]`` here so
+        the module stays importable in environments where ``textual`` is
+        not installed (typecheck gate, pure-domain tests).
+        """
         with contextlib.suppress(Exception):  # pragma: no cover
             Static = importlib.import_module("textual.widgets").Static
             yield Static(self._text, id="permissions-content")