feat(mcp): add MCP JSON renderer and CLI support, update tests accordingly

- Add MCPRenderer to generate MCP-native JSON bundles (index.json, nav.json, modules/*.json)
- Expose MCPRenderer via public API and CLI (`generate-mcp` command)
- Replace Markdown-based MCP output with structured JSON resources
- Update MCP renderer type stubs to match new JSON-based implementation
- Refactor MCP tests to validate JSON content, bundle structure, and navigation
- Fix MCP module coverage test to use explicit project_root for reliable discovery

tests/renderers/mcp/test_mcp_idempotency.py

@@ -12,9 +12,9 @@ def test_mcp_idempotent(tmp_path: Path):
     renderer = MCPRenderer()
 
     renderer.generate_sources(project, out_dir)
-    first = (out_dir / "testpkg" / "mod.md").read_text()
+    first = (out_dir / "modules" / "testpkg.mod.json").read_text()
 
     renderer.generate_sources(project, out_dir)
-    second = (out_dir / "testpkg" / "mod.md").read_text()
+    second = (out_dir / "modules" / "testpkg.mod.json").read_text()
 
     assert first == second