* feat(extensions): add example guide and packaging checks
* fix(extensions): align packaging checks with installer
Reject Windows drive-qualified wheel paths before extraction and reuse the installer entry-point validator. Cover unsafe paths, dotted attributes, and valid dotted modules in regression tests.
* fix(extensions): skip unused wheel extraction
Only extract wheels when browser manifests need validation, with a backend-only regression test. Link the three remaining example READMEs in the developer guide.
* docs(frontend): trim guidance intro below size budget
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>