feat(addon-system): create kindred-addon-sdk package (#249)

Add mods/sdk/ with the kindred_sdk Python package providing a stable
API layer for addon integration with Kindred Create platform features.

Modules:
- context: editing context/overlay registration wrappers
- theme: YAML-driven palette system (Catppuccin Mocha)
- origin: FileOrigin registration helpers
- dock: deferred dock panel registration
- compat: version detection utilities

The SDK loads at priority 0 (before all other addons) via the existing
manifest-driven loader. Theme colors are defined in a single YAML
palette file instead of hardcoded Python dicts, enabling future theme
support and eliminating color duplication across addons.

Closes #249

src/Mod/Create/CMakeLists.txt

@@ -54,3 +54,19 @@ install(
     DESTINATION
         mods/silo/silo-client
 )
+
+# Install SDK
+install(
+    DIRECTORY
+        ${CMAKE_SOURCE_DIR}/mods/sdk/kindred_sdk
+    DESTINATION
+        mods/sdk
+)
+install(
+    FILES
+        ${CMAKE_SOURCE_DIR}/mods/sdk/package.xml
+        ${CMAKE_SOURCE_DIR}/mods/sdk/Init.py
+        ${CMAKE_SOURCE_DIR}/mods/sdk/InitGui.py
+    DESTINATION
+        mods/sdk
+)