fix: correct verify-build diagnostics and close two robustness gaps (closes #180)

The both-markers diagnostic claimed the debug branch was still live. It is
not: with the __BUILD_DEBUG__ define removed, the emitted bundle carries
`typeof __BUILD_DEBUG__<"u"?__BUILD_DEBUG__:!1`, and in extension context the
identifier is undeclared, so DEBUG evaluates to false at runtime. The message
now states what the check does prove -- DEBUG was not resolved at build time,
so the release/debug distinction is no longer enforced and which way the
unresolved fallback evaluates is an accident a refactor can flip -- and it
remains a hard failure. The other seven failure messages were reviewed and
none needed rewording.

has_marker no longer swallows grep's exit 2 with 2>/dev/null. Match and
no-match are answers about the emitted output; an unreadable file is not, and
is now reported as a permissions or I/O fault instead of as "the emitted
output changed shape". Both paths still fail hard.

The unlisted-bundle scan no longer filters by extension, so the endsWith(".js")
test in build.js is the only place that assumption lives. A bundle emitted
under another extension previously escaped the manifest and the cross-check at
once; it now fails as unlisted. Both sites carry a comment naming the other.

Also: the manifest must be readable and a listed bundle must be non-empty,
so a vacuous input fails loudly rather than reaching a marker check that
cannot prove anything.

build.js

@@ -29,6 +29,11 @@ function repoRelative(p) {
 // reports every input that contributed to an output in the metafile, which is
 // the authoritative answer to "is constants.js in this bundle" — unlike
 // searching the minified text, it does not depend on what survived minification.
+//
+// The ".js" filter below is the only place that assumption lives:
+// script/verify-build searches every file under dist/ for a marker, without
+// filtering by extension, so a bundle emitted under some other extension fails
+// there as unlisted rather than escaping both checks at once.
 function outputsContainingAuditedModule(metafile) {
     return Object.entries(metafile.outputs)
         .filter(([outFile, info]) => {