PR #82622 fresh before/after proof

Behavior: bundled generated entry resolution should prefer packaged dist-runtime/extensions before falling back to source extensions.

BEFORE upstream/main:
status=0
resolved=extensions/plugin/index.ts
usesDistRuntime=false
sourceFallback=true
exists=true

AFTER PR #82622 head 3e53c8bb8165e01a79f9f43c6530f1eec41efaba:
status=0
resolved=dist-runtime/extensions/plugin/index.js
usesDistRuntime=true
sourceFallback=false
exists=true