fix: Correct inlet/outlet types for all stages and add comprehensive tests

Fixes and improvements:

1. Corrected Stage Type Declarations
   - DataSourceStage: NONE inlet, SOURCE_ITEMS outlet (was incorrectly set to TEXT_BUFFER)
   - CameraStage: TEXT_BUFFER inlet/outlet (post-render transformation, was SOURCE_ITEMS)
   - All other stages correctly declare their inlet/outlet types
   - ImageToTextStage: Removed unused ImageItem import

2. Test Suite Organization
   - Moved TestInletOutletTypeValidation class to proper location
   - Added pytest and DataType/StageError imports to test file header
   - Removed duplicate imports
   - All 5 type validation tests passing

3. Type Validation Coverage
   - Type mismatch detection raises StageError at build time
   - Compatible types pass validation
   - DataType.ANY accepts everything
   - Multiple inlet types supported
   - Display stage restrictions enforced

All data flows now properly validated:
- Source (SOURCE_ITEMS) → Render (TEXT_BUFFER) → Effects/Camera (TEXT_BUFFER) → Display

Tests: 507 tests passing

engine/pipeline/adapters.py

@@ -56,6 +56,18 @@ class EffectPluginStage(Stage):
     def dependencies(self) -> set[str]:
         return set()
 
+    @property
+    def inlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.TEXT_BUFFER}
+
+    @property
+    def outlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.TEXT_BUFFER}
+
     def process(self, data: Any, ctx: PipelineContext) -> Any:
         """Process data through the effect."""
         if data is None:
@@ -113,6 +125,18 @@ class DisplayStage(Stage):
     def dependencies(self) -> set[str]:
         return {"render.output"}  # Display needs rendered content
 
+    @property
+    def inlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.TEXT_BUFFER}  # Display consumes rendered text
+
+    @property
+    def outlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.NONE}  # Display is a terminal stage (no output)
+
     def init(self, ctx: PipelineContext) -> bool:
         w = ctx.params.viewport_width if ctx.params else 80
         h = ctx.params.viewport_height if ctx.params else 24
@@ -146,6 +170,18 @@ class DataSourceStage(Stage):
     def dependencies(self) -> set[str]:
         return set()
 
+    @property
+    def inlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.NONE}  # Sources don't take input
+
+    @property
+    def outlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.SOURCE_ITEMS}
+
     def process(self, data: Any, ctx: PipelineContext) -> Any:
         """Fetch data from source."""
         if hasattr(self._source, "get_items"):
@@ -177,6 +213,18 @@ class PassthroughStage(Stage):
     def dependencies(self) -> set[str]:
         return {"source"}
 
+    @property
+    def inlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.SOURCE_ITEMS}
+
+    @property
+    def outlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.SOURCE_ITEMS}
+
     def process(self, data: Any, ctx: PipelineContext) -> Any:
         """Pass data through unchanged."""
         return data
@@ -206,6 +254,18 @@ class SourceItemsToBufferStage(Stage):
     def dependencies(self) -> set[str]:
         return {"source"}
 
+    @property
+    def inlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.SOURCE_ITEMS}
+
+    @property
+    def outlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.TEXT_BUFFER}
+
     def process(self, data: Any, ctx: PipelineContext) -> Any:
         """Convert SourceItem list to text buffer."""
         if data is None:
@@ -258,6 +318,18 @@ class CameraStage(Stage):
             "source"
         }  # Prefix match any source (source.headlines, source.poetry, etc.)
 
+    @property
+    def inlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.TEXT_BUFFER}  # Camera works on rendered text
+
+    @property
+    def outlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.TEXT_BUFFER}
+
     def process(self, data: Any, ctx: PipelineContext) -> Any:
         """Apply camera transformation to data."""
         if data is None:
@@ -317,6 +389,18 @@ class FontStage(Stage):
     def dependencies(self) -> set[str]:
         return {"source"}
 
+    @property
+    def inlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.SOURCE_ITEMS}
+
+    @property
+    def outlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.TEXT_BUFFER}
+
     def init(self, ctx: PipelineContext) -> bool:
         """Initialize font from config or path."""
         from engine import config
@@ -404,6 +488,12 @@ class ImageToTextStage(Stage):
     def stage_type(self) -> str:
         return "transform"
 
+    @property
+    def inlet_types(self) -> set:
+        from engine.pipeline.core import DataType
+
+        return {DataType.PIL_IMAGE}  # Accepts PIL Image objects or ImageItem
+
     @property
     def outlet_types(self) -> set:
         from engine.pipeline.core import DataType