The bun:bundle module only exists at bundle time, not when running source
directly (dev/tests). This caused CI failures with:
error: Cannot find package 'bundle'
Fix: Use Bun.build's define option to set __USE_MAGICK__ as a compile-time
constant. At dev/test time it's undefined (defaults to false/sharp).
At build time it's set based on USE_MAGICK env var.
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