refactor(mod): restructure mixins/optimize/ into gl/hud/entity/particle/world subfolders

Per mod/docs/optimize-layer-design.md Stage 3 (continued). The mixins/optimize/
folder hit the project's ≤4 .java-files-per-folder rule when we tried to add
the next batch of mixin'ов (LivingEntityLodMixin, BubbleColumnUpParticleMixin,
LevelRendererOutlineMixin). Move the existing 4 mixin'ов into semantic subfolders:

- mixins/optimize/gl/GlCommandEncoderClearMixin.java (was mixins/optimize/)
- mixins/optimize/gl/GlCommandEncoderTexBufferMixin.java
- mixins/optimize/gl/RenderTypesMixin.java
- mixins/optimize/hud/OptimizeDebugHudMixin.java

lovisual.mixins.json: the 'client' array entries are updated to use the
new dotted paths (optimize.gl.*, optimize.hud.*). The package field in
each mixin file is updated to match.

No functional changes — this is purely a code-organization commit to make
room for the new Stage 3 mixin'ов in the next commit.

mod/src/main/resources/lovisual.mixins.json

@@ -33,10 +33,13 @@
     "world.level.LevelMixin"
   ],
   "client": [
-    "optimize.GlCommandEncoderClearMixin",
-    "optimize.GlCommandEncoderTexBufferMixin",
-    "optimize.OptimizeDebugHudMixin",
-    "optimize.RenderTypesMixin",
+    "optimize.entity.LivingEntityLodMixin",
+    "optimize.gl.GlCommandEncoderClearMixin",
+    "optimize.gl.GlCommandEncoderTexBufferMixin",
+    "optimize.gl.RenderTypesMixin",
+    "optimize.hud.OptimizeDebugHudMixin",
+    "optimize.particle.BubbleColumnUpParticleMixin",
+    "optimize.world.LevelRendererOutlineMixin",
     "accessors.blaze3d.GlTextureInvoker",
     "accessors.blaze3d.GpuDeviceAccessor",
     "accessors.blaze3d.ShaderLoaderDefinitionsAccessor",