Add item icon art for all 41 item types

Self-contained full-color SVGs in data/icons/items/, one per item id, read
by the recipe-selection dialog (REQ-UI-RECIPE-ICON) and belt/port rendering
(REQ-UI-ITEM-ICON). Bold silhouette + dark outline, authored to stay legible
at ~16px belt size: rocky ore chunks, isometric ingots, riveted plates,
copper coil/wire, pictorial module devices, and dart hull silhouettes tinted
per fleet color. No config changes needed — each recipe's icon resolves to
its first output.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

bin/app/data/icons/items/hardened_steel.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" width="100" height="100">
+  <rect x="20" y="26" width="60" height="48" rx="6" fill="#6a7280" stroke="#181c22" stroke-width="4"/>
+  <rect x="29" y="35" width="42" height="30" rx="3" fill="#7c8593" stroke="#181c22" stroke-width="2.5"/>
+  <g fill="#2a2f38">
+    <circle cx="26" cy="32" r="3.2"/><circle cx="74" cy="32" r="3.2"/>
+    <circle cx="26" cy="68" r="3.2"/><circle cx="74" cy="68" r="3.2"/>
+  </g>
+</svg>