Add bounded portable Home thumbnails to design files (#518)

A save with cache: true also stores a small bounded PNG thumbnail (240 × 150, at most 256 KiB, keyed to the exact document) made from cached meshes only. Home uses it when its local picture is missing, without rebuilding the design; ordinary saves and downloads still omit it. Closes #303.

Co-authored-by: lillith <[email protected]>

ROADMAP.md

@@ -389,6 +389,10 @@ and reference-object placement remain separate work, as do loft rails and center
   evaluation or meshing; explicit `preview: true` remains a separate warm-mesh option.
   Keys, resource budgets and document/revision guards reject incompatible, hostile or
   stale results. A second modelling kernel still requires its own tested adapter.
+- Warm saves with explicit `cache: true` can also carry a bounded PNG thumbnail from already cached meshes.
+  Home reads it without rebuilding the design when its local picture is missing;
+  mismatched or malformed thumbnails fall back to normal generation. The image is
+  kernel-independent and saving never forces cold meshing to produce it.
 
 - Cold rebuild profiling separates parsing, geometry, placement, display meshing and
   thumbnails. Straight-line closest-point projection and exact curved-surface evaluation