fix(api): only apply libc filter when caller pinned a meaningful libc

filterReleases unconditionally rejected libc=musl entries unless the
host was libc=musl, even when the caller never specified a libc in
the request. serve-releases.js defaults the libc parameter to 'libc'
(the catch-all glibc-host bucket the installer-side resolver uses),
so the website's release table and the WEBI_RELEASES probe were both
stripped of every musl entry that the cache actually contained — even
though the installer would happily consider those builds on a glibc
host (its waterfall is [none, gnu, musl, libc]).

Treat libc='libc' (and missing) as 'no preference' so the filter only
runs when the caller pinned a real libc (musl, gnu, msvc, etc.).
Specific-libc queries (?libc=musl, ?libc=gnu) still filter exactly as
before.

_webi/transform-releases.js

@@ -112,7 +112,10 @@ async function filterReleases(
       }
     }
 
-    if (rel.libc !== 'none') {
+    // libc='libc' is serve-releases.js's default when the caller
+    // didn't pin one — treat it as 'no preference', not a filter.
+    let isMeaningfulLibc = libc && libc !== 'libc' && rel.libc !== 'none';
+    if (isMeaningfulLibc) {
       let releaseRequiresMusl = rel.libc === 'musl';
       // goal: handle non-glibc (Alpine / Docker / musl)
       let osHasMusl = libc === 'musl';