feat(pipeline): add PureData-style inlet/outlet typing

- Add DataType enum (SOURCE_ITEMS, TEXT_BUFFER, etc.)
- Add inlet_types and outlet_types to Stage
- Add _validate_types() for type checking at build time
- Update tests with proper type annotations

engine/pipeline/core.py

@@ -5,17 +5,40 @@ This module provides the foundation for a clean, dependency-managed pipeline:
 - Stage: Base class for all pipeline components (sources, effects, displays, cameras)
 - PipelineContext: Dependency injection context for runtime data exchange
 - Capability system: Explicit capability declarations with duck-typing support
+- DataType: PureData-style inlet/outlet typing for validation
 """
 
 from abc import ABC, abstractmethod
 from collections.abc import Callable
 from dataclasses import dataclass, field
+from enum import Enum, auto
 from typing import TYPE_CHECKING, Any
 
 if TYPE_CHECKING:
     from engine.pipeline.params import PipelineParams
 
 
+class DataType(Enum):
+    """PureData-style data types for inlet/outlet validation.
+
+    Each type represents a specific data format that flows through the pipeline.
+    This enables compile-time-like validation of connections.
+
+    Examples:
+        SOURCE_ITEMS: List[SourceItem] - raw items from sources
+        ITEM_TUPLES: List[tuple] - (title, source, timestamp) tuples
+        TEXT_BUFFER: List[str] - rendered ANSI buffer for display
+        RAW_TEXT: str - raw text strings
+    """
+
+    SOURCE_ITEMS = auto()  # List[SourceItem] - from DataSource
+    ITEM_TUPLES = auto()  # List[tuple] - (title, source, ts)
+    TEXT_BUFFER = auto()  # List[str] - ANSI buffer
+    RAW_TEXT = auto()  # str - raw text
+    ANY = auto()  # Accepts any type
+    NONE = auto()  # No data (terminator)
+
+
 @dataclass
 class StageConfig:
     """Configuration for a single stage."""
@@ -35,18 +58,78 @@ class Stage(ABC):
     - Effects: Post-processors (noise, fade, glitch, hud)
     - Displays: Output backends (terminal, pygame, websocket)
     - Cameras: Viewport controllers (vertical, horizontal, omni)
+    - Overlays: UI elements that compose on top (HUD)
 
     Stages declare:
     - capabilities: What they provide to other stages
     - dependencies: What they need from other stages
+    - stage_type: Category of stage (source, effect, overlay, display)
+    - render_order: Execution order within category
+    - is_overlay: If True, output is composited on top, not passed downstream
 
     Duck-typing is supported: any class with the required methods can act as a Stage.
     """
 
     name: str
-    category: str  # "source", "effect", "display", "camera"
+    category: str  # "source", "effect", "overlay", "display", "camera"
     optional: bool = False  # If True, pipeline continues even if stage fails
 
+    @property
+    def stage_type(self) -> str:
+        """Category of stage for ordering.
+
+        Valid values: "source", "effect", "overlay", "display", "camera"
+        Defaults to category for backwards compatibility.
+        """
+        return self.category
+
+    @property
+    def render_order(self) -> int:
+        """Execution order within stage_type group.
+
+        Higher values execute later. Useful for ordering overlays
+        or effects that need specific execution order.
+        """
+        return 0
+
+    @property
+    def is_overlay(self) -> bool:
+        """If True, this stage's output is composited on top of the buffer.
+
+        Overlay stages don't pass their output to the next stage.
+        Instead, their output is layered on top of the final buffer.
+        Use this for HUD, status displays, and similar UI elements.
+        """
+        return False
+
+    @property
+    def inlet_types(self) -> set[DataType]:
+        """Return set of data types this stage accepts.
+
+        PureData-style inlet typing. If the connected upstream stage's
+        outlet_type is not in this set, the pipeline will raise an error.
+
+        Examples:
+            - Source stages: {DataType.NONE} (no input needed)
+            - Transform stages: {DataType.ITEM_TUPLES, DataType.TEXT_BUFFER}
+            - Display stages: {DataType.TEXT_BUFFER}
+        """
+        return {DataType.ANY}
+
+    @property
+    def outlet_types(self) -> set[DataType]:
+        """Return set of data types this stage produces.
+
+        PureData-style outlet typing. Downstream stages must accept
+        this type in their inlet_types.
+
+        Examples:
+            - Source stages: {DataType.SOURCE_ITEMS}
+            - Transform stages: {DataType.TEXT_BUFFER}
+            - Display stages: {DataType.NONE} (consumes data)
+        """
+        return {DataType.ANY}
+
     @property
     def capabilities(self) -> set[str]:
         """Return set of capabilities this stage provides.