Build 1.21.7 with Stonecutter from the shared src/

Step 5 of #8. versions/1.21.7/src is gone; 1.21.7 builds from src/ as
:1.21.7.

1.21.7's copy differed from 1.21.8's in the player render state: Fabric
API's RenderStateDataKey only exists from 1.21.8 (fabric-rendering-v1
12.6.0), so 1.21.7 had no updateRenderState hook, no SaturnRenderState,
and looked players up by name for the cloak and hat. That stays the case:
- The cloak and hat use getData on >=1.21.8 and SaturnPlayer.get(name)
  before that.
- PlayerEntityRendererMixin's updateRenderState exists only on >=1.21.8,
  as flat ranges (>=1.21.9, else if >=1.21.8) calling a shared
  saturn$applyPlayer helper, instead of nested version blocks.
- SaturnRenderState's key only exists on >=1.21.8; the class is empty
  before.
- TODO: 1.21.6 and 1.21.7 still don't show the role icon in names.

1.21.7's NameTagMixin isn't carried over: it wasn't registered in any
mixin config, so it never ran.

The 1.21.7 jar differs from the old one only as 1.21.8's did in step 4
(entityAlpha and drawGroup helpers, getIconId, and the cloak getting its
buffer at the draw call), plus the empty SaturnRenderState and the
missing dead NameTagMixin. 1.21.8-1.21.11 only differ in
PlayerEntityRendererMixin (the helper).

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

README.MD

@@ -47,7 +47,7 @@ git clone --recurse-submodules https://git.selimaj.dev/saturnclientmc/saturnclie
 cd saturnclient
 
 ./gradlew buildAll                    # every version → build/allJars/
-./gradlew :1.21.11:build              # a single version (versions not on Stonecutter yet: :mc-1.21.7)
+./gradlew :1.21.11:build              # a single version (versions not on Stonecutter yet: :mc-1.21.6)
 ./gradlew :1.21.11:runClient          # launch a development client
 ```