Recolour the real heart of the sea for the arcana core

The texture was drawn to that silhouette rather than taken from it,
because there was no copy of the vanilla assets on hand. There is now:
the item texture is Minecraft's own heart of the sea with its hue
rotated from cyan into violet and nothing else touched, so the shading
is the vanilla item's.

That makes it the one piece of art here that is not drawn for this mod,
which the README now says next to the khopesh paragraph that takes the
opposite line. It also means the palette the material hands out is a
deep one with almost no highlight - the heart of the sea is a dark item,
and the sampler reports what a texture looks like rather than re-lighting
it.

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

README.md

@@ -159,8 +159,13 @@ way a datapack can. Nothing that makes it worth having is: the material and the
 recipe both live in the Iron's Spells pack, so without that mod the core cannot
 be crafted, is not a material, and grants nothing.
 
-Its texture is the heart of the sea's shape in purple, drawn here, and
+Its texture is Minecraft's own heart of the sea with its hue rotated into
-placeholder-grade like the rest of the art.
+violet - the only art here that is not drawn for this mod, and Mojang's rather
+than ours, which is worth knowing given what the khopesh section above says
+about shipping other people's. Nothing but the hue is touched, so the shading
+is the vanilla item's, including how dark its underside is: the palette the
+material hands out is therefore a deep one with little highlight, which is what
+the item actually looks like rather than what a gem usually does.
 
 ## Stats
 

src/main/resources/packs/irons_spellbooks/data/cmmodular/miapi/materials/crystal/arcana_core.json

@@ -21,17 +21,17 @@
   "mining_level": "minecraft:incorrect_for_netherite_tool",
   "armor_durability": 48,
   "armor_toughness": 3.0,
-  "color": "CD94EF",
+  "color": "711CA9",
   "color_palette": {
     "type": "grayscale_map",
     "colors": {
-      "24": "13061d",
+      "24": "1c042a",
-      "68": "250b38",
+      "68": "350851",
-      "107": "250b38",
+      "107": "4d0458",
-      "150": "8637ba",
+      "150": "500a7b",
-      "190": "a95cdb",
+      "190": "7319ba",
-      "216": "cd94ef",
+      "216": "711ca9",
-      "255": "f0dcff"
+      "255": "711ca9"
     },
     "filler": "interpolate"
   },