feat: add dev preview server, CLI render script, and visual test display manager

Adds local development tools for rapid plugin iteration without deploying to RPi:

- VisualTestDisplayManager: renders real pixels via PIL (same fonts/interface as production)
- Dev preview server (Flask): interactive web UI with plugin picker, auto-generated config
  forms, zoom/grid controls, and mock data support for API-dependent plugins
- CLI render script: render any plugin to PNG for AI-assisted visual feedback loops
- Updated test runner and conftest to auto-detect plugin-repos/ directory

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/render_plugin.py

@@ -0,0 +1,185 @@
+#!/usr/bin/env python3
+"""
+Plugin Visual Renderer
+
+Loads a plugin, calls update() + display(), and saves the resulting
+display as a PNG image for visual inspection.
+
+Usage:
+    python scripts/render_plugin.py --plugin hello-world --output /tmp/hello.png
+    python scripts/render_plugin.py --plugin clock-simple --plugin-dir plugin-repos/ --output /tmp/clock.png
+    python scripts/render_plugin.py --plugin hello-world --config '{"message":"Test!"}' --output /tmp/test.png
+    python scripts/render_plugin.py --plugin football-scoreboard --mock-data mock_scores.json --output /tmp/football.png
+"""
+
+import sys
+import os
+import json
+import argparse
+from pathlib import Path
+
+# Add project root to path
+PROJECT_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+# Prevent hardware imports
+os.environ['EMULATOR'] = 'true'
+
+
+def find_plugin_dir(plugin_id, search_dirs):
+    """Find a plugin directory by searching multiple paths."""
+    from src.plugin_system.plugin_loader import PluginLoader
+    loader = PluginLoader()
+    for search_dir in search_dirs:
+        search_path = Path(search_dir)
+        if not search_path.exists():
+            continue
+        result = loader.find_plugin_directory(plugin_id, search_path)
+        if result:
+            return result
+    return None
+
+
+def load_manifest(plugin_dir):
+    """Load and return manifest.json from plugin directory."""
+    manifest_path = plugin_dir / 'manifest.json'
+    if not manifest_path.exists():
+        raise FileNotFoundError(f"No manifest.json in {plugin_dir}")
+    with open(manifest_path, 'r') as f:
+        return json.load(f)
+
+
+def load_config_defaults(plugin_dir):
+    """Extract default values from config_schema.json."""
+    schema_path = plugin_dir / 'config_schema.json'
+    if not schema_path.exists():
+        return {}
+    with open(schema_path, 'r') as f:
+        schema = json.load(f)
+    defaults = {}
+    for key, prop in schema.get('properties', {}).items():
+        if 'default' in prop:
+            defaults[key] = prop['default']
+    return defaults
+
+
+def main():
+    parser = argparse.ArgumentParser(description='Render a plugin display to a PNG image')
+    parser.add_argument('--plugin', '-p', required=True, help='Plugin ID to render')
+    parser.add_argument('--plugin-dir', '-d', default=None,
+                        help='Directory to search for plugins (default: auto-detect)')
+    parser.add_argument('--config', '-c', default='{}',
+                        help='Plugin config as JSON string')
+    parser.add_argument('--mock-data', '-m', default=None,
+                        help='Path to JSON file with mock cache data')
+    parser.add_argument('--output', '-o', default='/tmp/plugin_render.png',
+                        help='Output PNG path (default: /tmp/plugin_render.png)')
+    parser.add_argument('--width', type=int, default=128, help='Display width (default: 128)')
+    parser.add_argument('--height', type=int, default=32, help='Display height (default: 32)')
+    parser.add_argument('--skip-update', action='store_true',
+                        help='Skip calling update() (render display only)')
+
+    args = parser.parse_args()
+
+    # Determine search directories
+    if args.plugin_dir:
+        search_dirs = [args.plugin_dir]
+    else:
+        search_dirs = [
+            str(PROJECT_ROOT / 'plugins'),
+            str(PROJECT_ROOT / 'plugin-repos'),
+        ]
+
+    # Find plugin
+    plugin_dir = find_plugin_dir(args.plugin, search_dirs)
+    if not plugin_dir:
+        print(f"Error: Plugin '{args.plugin}' not found in: {search_dirs}")
+        return 1
+
+    print(f"Found plugin at: {plugin_dir}")
+
+    # Load manifest
+    manifest = load_manifest(Path(plugin_dir))
+
+    # Parse config: start with schema defaults, then apply overrides
+    config_defaults = load_config_defaults(Path(plugin_dir))
+    try:
+        user_config = json.loads(args.config)
+    except json.JSONDecodeError as e:
+        print(f"Error: Invalid JSON config: {e}")
+        return 1
+
+    config = {'enabled': True}
+    config.update(config_defaults)
+    config.update(user_config)
+
+    # Load mock data if provided
+    mock_data = {}
+    if args.mock_data:
+        mock_data_path = Path(args.mock_data)
+        if not mock_data_path.exists():
+            print(f"Error: Mock data file not found: {args.mock_data}")
+            return 1
+        with open(mock_data_path, 'r') as f:
+            mock_data = json.load(f)
+
+    # Create visual display manager and mocks
+    from src.plugin_system.testing import VisualTestDisplayManager, MockCacheManager, MockPluginManager
+    from src.plugin_system.plugin_loader import PluginLoader
+
+    display_manager = VisualTestDisplayManager(width=args.width, height=args.height)
+    cache_manager = MockCacheManager()
+    plugin_manager = MockPluginManager()
+
+    # Pre-populate cache with mock data
+    for key, value in mock_data.items():
+        cache_manager.set(key, value)
+
+    # Load and instantiate plugin
+    loader = PluginLoader()
+
+    try:
+        plugin_instance, module = loader.load_plugin(
+            plugin_id=args.plugin,
+            manifest=manifest,
+            plugin_dir=Path(plugin_dir),
+            config=config,
+            display_manager=display_manager,
+            cache_manager=cache_manager,
+            plugin_manager=plugin_manager,
+            install_deps=True,
+        )
+    except Exception as e:
+        print(f"Error loading plugin: {e}")
+        return 1
+
+    print(f"Plugin '{args.plugin}' loaded successfully")
+
+    # Run update() then display()
+    if not args.skip_update:
+        try:
+            plugin_instance.update()
+            print("update() completed")
+        except Exception as e:
+            print(f"Warning: update() raised: {e}")
+            print("Continuing to display()...")
+
+    try:
+        plugin_instance.display(force_clear=True)
+        print("display() completed")
+    except Exception as e:
+        print(f"Error in display(): {e}")
+        return 1
+
+    # Save the rendered image
+    output_path = Path(args.output)
+    output_path.parent.mkdir(parents=True, exist_ok=True)
+    display_manager.save_snapshot(str(output_path))
+    print(f"Rendered image saved to: {output_path}")
+    print(f"Dimensions: {args.width}x{args.height}")
+
+    return 0
+
+
+if __name__ == '__main__':
+    sys.exit(main())