[no bump] Move technical docs to docs/, add icon PNG export tool

README now carries only the icon, TL;DR, description and features; the
technical sections live in docs/. tools/icon_png.py renders
images/icon.svg to a square PNG (512x512 by default, --resolution N).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

images/icon.svg

@@ -0,0 +1,35 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="640" height="640" viewBox="0 0 640 640">
+  <defs>
+    <linearGradient id="bg" x1="0.3" y1="0" x2="0.7" y2="1">
+      <stop offset="0" stop-color="#F89B2E"/><stop offset="0.5" stop-color="#F7724A"/><stop offset="1" stop-color="#F5517A"/>
+    </linearGradient>
+    <radialGradient id="glow" cx="0.5" cy="0.45" r="0.5">
+      <stop offset="0" stop-color="#FF5A3C" stop-opacity="0.18"/><stop offset="1" stop-color="#FF5A3C" stop-opacity="0"/>
+    </radialGradient>
+    <linearGradient id="paper" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#F7F7F7"/><stop offset="1" stop-color="#E2E2E2"/>
+    </linearGradient>
+    <linearGradient id="star" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#FFE27A"/><stop offset="1" stop-color="#FFC53D"/>
+    </linearGradient>
+    <linearGradient id="sky" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#7CC8FF"/><stop offset="1" stop-color="#CFEBFF"/>
+    </linearGradient>
+    <linearGradient id="metal" x1="0" y1="0" x2="1" y2="0">
+      <stop offset="0" stop-color="#9C9C9C"/><stop offset="0.45" stop-color="#E4E4E4"/><stop offset="1" stop-color="#A8A8A8"/>
+    </linearGradient>
+    <clipPath id="photo"><rect x="166" y="196" width="228" height="208" rx="18"/></clipPath>
+    <filter id="sh" x="-20%" y="-20%" width="140%" height="150%">
+      <feDropShadow dx="0" dy="10" stdDeviation="10" flood-color="#7A1E2E" flood-opacity="0.28"/>
+    </filter>
+  </defs>
+  <rect width="640" height="640" fill="url(#bg)"/>
+  <rect width="640" height="640" fill="url(#glow)"/>
+
+  <g transform="translate(0,-14)">
+    <rect filter="url(#sh)" x="140" y="470" width="360" height="54" rx="20" fill="url(#paper)" stroke="#B9B9B9" stroke-width="6" stroke-linejoin="round"/>
+    <line x1="176" y1="497" x2="464" y2="497" stroke="#CFCFCF" stroke-width="6" stroke-linecap="round"/>
+    <path filter="url(#sh)" transform="translate(-32,0)" fill="url(#star)" stroke="#F59A2E" stroke-width="8" stroke-linejoin="round" d="M200,220 H285 C275,180 290,152 320,152 C350,152 365,180 355,220 H440 V300 C480,290 508,305 508,340 C508,375 480,390 440,380 V460 H355 C365,420 350,398 320,398 C290,398 275,420 285,460 H200 Z"/>
+    <path transform="translate(-32,0)" d="M222,250 V300" stroke="#FFF3C4" stroke-width="12" stroke-linecap="round"/>
+  </g>
+</svg>