fix: avoid body stream already read on fallback

[rinarjn]

Fix “body stream already read” & Playground stall after switching to jsDelivr.

Why it appeared after switching CDNs:

• unpkg often didn’t send Content-Length, so strict byte check never ran.
• jsDelivr sends a compressed response with Content-Length. Our progress counts decoded bytes, which don’t match the compressed length, so the fallback tried to read the same Response twice.
• In dev, React StrictMode double-invocation makes this more likely to surface. In prod, it stalls the Playground around 90–98%.
• Fix: read the fallback from a cloned Response.

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[lucasgelfond]

amazing ty for catching!!