feat(diagram-render): __downscaleRaster for print-resolution image normalization

Data-URI rasters re-encode in their own format (JPEG stays JPEG at q0.9 —
PNG-encoding photos bloats them) at an explicit target pixel width. Used by
make-pdf's pre-pass for the 300dpi content-box ceiling (eng-review D4).

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

lib/diagram-render/dist/BUILD_INFO.json

@@ -1,7 +1,7 @@
 {
   "name": "gstack-diagram-render",
-  "sha256": "9b1da250ef93f176bfb7b96c6eb735645a879702539bb6a58c378273d90aca23",
-  "bytes": 9644924,
+  "sha256": "0ee91aef5a8da85c8941c26ebf2991bbeba82412644bb070d5c5dd2e23538b81",
+  "bytes": 9645503,
   "bunVersion": "1.3.13",
   "deps": {
     "@excalidraw/excalidraw": "0.18.0",