Add tests for image layer, image plane, inventory, saved layout, commands, documentation, manifest, and packaging

- Implement tests for image layer functionality including alpha curve and depth layer behavior.
- Create tests for image plane evaluation and modifier input handling.
- Add tests to verify the saved asset inventory and its persistence.
- Introduce tests for the documentation command to ensure correct project version deployment.
- Validate the extension manifest structure and required permissions.
- Develop tests for the packaging process, ensuring correct platform handling and dependency management.
- Enhance image depth plane group functionality with new split threshold input and surface processing.

tests/conftest.py

@@ -1,5 +1,5 @@
-"""Expose project source packages to the test suite."""
+"""Expose the extension and standalone server packages to the test suite."""
 import sys
-from pathlib import Path
+from tests.support.paths import PROJECT_ROOT
 
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))
+sys.path[:0] = [str(PROJECT_ROOT / "src"), str(PROJECT_ROOT / "src/anyimage")]