refactor(deprecate): Add deprecation warning to RenderStage (Phase 4.1)

- Add DeprecationWarning to RenderStage.__init__()
- Document that RenderStage uses legacy rendering code
- Recommend modern pipeline stages as replacement
- ItemsStage already has deprecation warning
- Tests pass (515 core tests, legacy failures pre-existing)

engine/pipeline/adapters.py

@@ -18,6 +18,11 @@ class RenderStage(Stage):
     - Selects headlines and renders them to blocks
     - Applies camera scroll position
     - Adds firehose layer if enabled
+
+    .. deprecated::
+        RenderStage uses legacy rendering from engine.legacy.layers and engine.legacy.render.
+        This stage will be removed in a future version. For new code, use modern pipeline stages
+        like PassthroughStage with custom rendering stages instead.
     """
 
     def __init__(
@@ -30,6 +35,15 @@ class RenderStage(Stage):
         firehose_enabled: bool = False,
         name: str = "render",
     ):
+        import warnings
+
+        warnings.warn(
+            "RenderStage is deprecated. It uses legacy rendering code from engine.legacy.*. "
+            "This stage will be removed in a future version. "
+            "Use modern pipeline stages with PassthroughStage or create custom rendering stages instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         self.name = name
         self.category = "render"
         self.optional = False