Extension loading iterated paths with await in a for...of loop, so each
module import (file I/O + module evaluation, the dominant cold-start
cost) blocked the next. Split loadExtension into a concurrent import
phase (Promise.all) and a sequential factory-binding phase that runs in
the original path order, keeping registration semantics (last-wins
collisions, shared runtime flag defaults) deterministic and per-extension
error isolation intact.
Fixes#7615