Placeholder bezels for the small and great cases

The medium case has had art of its own since the model adjustments - a bezel
around an empty socket, on the greys a material's palette maps - even though
the module wears MIAPI's gemstone model at the moment. The two new sizes had
nothing at all, so here are their blanks, drawn to the same rules: the top and
left edges catch the light, the right edge is the shadow, the bottom is the mid
tone, and the socket in the middle stays the darkest stop so it reads as empty
whatever is in it. Three pixels across for the small, six for the great, either
side of the medium's four.

They are placeholders in the sense the templates are: the right size, the right
palette, and something to draw over rather than finished art. Each gets the
model beside it that the medium one has, so wiring a case to its own picture is
a path change in the module and nothing else.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/main/resources/assets/cmmodular/models/item/armor/gems/case_great/default.json

@@ -0,0 +1,6 @@
+{
+  "parent": "item/generated",
+  "textures": {
+    "layer0": "cmmodular:items/gem_case_great"
+  }
+}

src/main/resources/assets/cmmodular/models/item/armor/gems/case_small/default.json

@@ -0,0 +1,6 @@
+{
+  "parent": "item/generated",
+  "textures": {
+    "layer0": "cmmodular:items/gem_case_small"
+  }
+}