check_unlisted_bundles read dist/ line by line, so two unlisted paths
carrying a debug marker went unchecked while the script still exited 0:
a name with a trailing space (read stripped it and the remnant matched a
manifest line) and a name containing a newline (find printed it as a
listed path plus an empty one).
The walk is now find -print0 into a temporary listing, checked for find's
exit status as before, and xargs -0 hands the paths back to this script as
arguments, where the same is_listed and has_marker run on them. is_listed
also answers "not listed" for any path containing a newline without asking
grep, which would otherwise read such a path as two patterns and match on
the first half — the escape survives NUL delimiting on its own.
dist/ being a symlink is now its own check with its own message. It failed
before only because GNU grep exits 2 on a directory, so a grep that exits 1
instead would have turned the whole cross-check into a pass.
The comment claiming every file under dist/ is searched now says what is
actually guaranteed: every regular file and every symlink, with the four
properties that coverage rests on stated as enforced rather than assumed.
build.js records which emitted bundles contain src/shared/constants.js, and
constants.js carries a marker constant-folded from DEBUG itself. script/verify-build
cross-checks the two and fails on every way of not knowing, so deleting the
__BUILD_DEBUG__ define now breaks the build instead of shipping a live debug branch.
