Build for Minecraft 26.1

Adds 26.1 as a Stonecutter version (step 3 of #39). Its jar also
loads on the 26.1.1 and 26.1.2 hotfixes.

Most of 26.1's changes are renames: the GUI's render methods became
extract methods and GuiGraphics became GuiGraphicsExtractor, which
Stonecutter replacements cover along with the moved render state
package and GuiRenderState's add methods. The rest are version
comments:

- The menu panorama is drawn by GuiRenderer from vanilla's cube map;
  the new GuiRendererMixin swaps in Saturn's while a Saturn screen is
  open (empty on older versions).
- The camera works out the FOV and aligns with its entity in
  alignWithEntity, so the zoom and freelook hooks move to CameraMixin.
- Gamma and the menu blur are read while extracting render state.
- The glass blur's post chain takes a Projection and its buffer.
- The dragon's alpha render type is gone; capes use entityCutout.
- Fabric API renamed KeyBindingHelper to KeyMappingHelper.

The docs describe the 26.x build and Java 25.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.MD

@@ -18,7 +18,7 @@
 Saturn Client is a free, open-source Minecraft client designed to enhance the gameplay experience while staying lightweight and customizable.
 Our goal is to build a clean, optimized client that the community can contribute to and improve together.
 
-Saturn runs on Fabric and supports **Minecraft 1.21.4 to 1.21.11**, with one build per version. If something doesn't work on your version, please open an issue.
+Saturn runs on Fabric and supports **Minecraft 1.21.4 to 1.21.11 and 26.1**, with one build per version (the 26.1 build also runs on 26.1.1 and 26.1.2). If something doesn't work on your version, please open an issue.
 
 ## Features
 
@@ -51,7 +51,7 @@ cd saturnclient
 ./gradlew :1.21.11:runClient          # launch a development client
 ```
 
-Building needs Java 21.
+Building needs Java 21. The 26.x versions compile with Java 25, which Gradle downloads when it isn't installed.
 
 ## Contributing