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/tools/test_docs.py

@@ -0,0 +1,42 @@
+from tests.support.paths import PROJECT_ROOT
+import importlib.util
+
+
+SCRIPT_PATH = PROJECT_ROOT / "tools" / "docs.py"
+SPEC = importlib.util.spec_from_file_location("docs_script", SCRIPT_PATH)
+docs = importlib.util.module_from_spec(SPEC)
+SPEC.loader.exec_module(docs)
+
+
+def test_documentation_command_deploys_project_version(monkeypatch):
+    monkeypatch.setattr(docs, "project_version", lambda: "1.2.3")
+
+    assert docs.documentation_command() == [
+        "mike",
+        "deploy",
+        "--update-aliases",
+        "1.2.3",
+        "latest",
+    ]
+
+
+def test_documentation_command_previews_current_source():
+    assert docs.documentation_command(dev=True) == ["mkdocs", "serve"]
+
+
+def test_main_runs_from_project_root(monkeypatch):
+    calls = []
+    monkeypatch.setattr(
+        docs.subprocess,
+        "run",
+        lambda *args, **kwargs: calls.append((args, kwargs)),
+    )
+
+    docs.main(["-dev"])
+
+    assert calls == [
+        (
+            (["mkdocs", "serve"],),
+            {"cwd": docs.PROJECT_ROOT, "check": True},
+        )
+    ]