feat(excalidraw): improve diagram quality with canvas measurement, validation, and conventions

Replace the charCount * fontSize * 0.55 text sizing heuristic with canvas-based
measurement (graceful fallback when native deps unavailable). Add validate.mjs for
automated spatial checks (text overflow, arrow-text collisions, element overlap).
Update element format reference with sizing rules, label guidelines, and arrow routing
conventions. Add verification step to SKILL.md workflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/compound-engineering/skills/excalidraw-png-export/scripts/setup.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+# First-time setup for excalidraw-png-export skill.
+# Installs playwright and chromium headless into a dedicated directory.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+EXPORT_DIR="$SCRIPT_DIR/.export-runtime"
+
+if [ -d "$EXPORT_DIR/node_modules/playwright" ]; then
+  echo "Runtime already installed at $EXPORT_DIR"
+  exit 0
+fi
+
+echo "Installing excalidraw-png-export runtime..."
+mkdir -p "$EXPORT_DIR"
+cd "$EXPORT_DIR"
+
+# Initialize package.json with ESM support
+cat > package.json << 'PACKAGEEOF'
+{
+  "name": "excalidraw-export-runtime",
+  "version": "1.0.0",
+  "type": "module",
+  "private": true
+}
+PACKAGEEOF
+
+npm install playwright 2>&1
+npx playwright install chromium 2>&1
+
+# canvas provides accurate text measurement for convert.mjs.
+# Requires Cairo native library: brew install pkg-config cairo pango libpng jpeg giflib librsvg
+# Falls back to heuristic sizing if unavailable.
+npm install canvas 2>&1 || echo "WARN: canvas install failed (missing Cairo?). Heuristic text sizing will be used."
+
+echo "Setup complete. Runtime installed at $EXPORT_DIR"