feat: render Markdown, mathematics and Mermaid diagrams server-side

internal/diagram/testdata/sq-break.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 348 244" font-family="sans-serif"><line x1="99" y1="68" x2="99" y2="220" stroke="#999" stroke-dasharray="5 5"/><line x1="249" y1="68" x2="249" y2="220" stroke="#999" stroke-dasharray="5 5"/><rect x="36" y="24" width="126" height="44" rx="8" fill="#eef2f7" stroke="#333" stroke-width="1.5"/><text x="99" y="51" text-anchor="middle" font-size="14" font-weight="bold">A</text><rect x="186" y="24" width="126" height="44" rx="8" fill="#eef2f7" stroke="#333" stroke-width="1.5"/><text x="249" y="51" text-anchor="middle" font-size="14" font-weight="bold">B</text><line x1="99" y1="166" x2="249" y2="166" stroke="#333" stroke-width="1.8"/><polygon points="249,166 238,161 238,171" fill="#333"/><text x="174" y="159" text-anchor="middle" font-size="13">stop</text><rect x="24" y="102" width="300" height="98" rx="6" fill="none" stroke="#888"/><rect x="24" y="102" width="120" height="22" fill="#eee" stroke="#888"/><text x="32" y="117" text-anchor="start" font-size="13">break failure</text></svg>