Fix CodePen export failing to inline blob assets on Safari

[apresmoi]

Summary

• exportPolySceneSnapshot inlines blob: atlas/texture assets by fetch()-ing them, which intermittently fails on WebKit/Safari ("Load failed") — so "Open in CodePen" reports could not inline asset "blob:…". This surfaces on mobile, where untextured solids fall to a single <s> atlas slice backed by a blob PNG (desktop uses <i> border-shape, no blob)
• add an XMLHttpRequest fallback for object URLs (blob:/filesystem:) when fetch() throws — the documented WebKit workaround — and drop the meaningless credentials mode for object-URL requests
• fetch() stays the primary path for all URLs, so non-Safari behavior is unchanged

Validation

• pnpm test && pnpm build
• new unit test: when fetch() rejects for a blob: URL, the exporter falls back to XHR and still produces the inlined data:image/png;base64,…