fix(#3527): a missing source must report as zero, not as empty

argus, reviewing, ran the guard against a source path that does not
exist rather than one that is empty. grep writes nothing to stdout in
that case, so `|| true` left the count variable empty and the -eq test
died with "integer expected" instead of reporting.

The unit still failed — cat hits the same missing file and set -e stops
it — but with a generic "no such file" rather than the message naming
which half is absent, which is the only thing this guard is for.

`|| echo 0` on all three counts. The gate gained the arm that was
missing: an absent source, asserted to fail THROUGH the guard rather
than merely to fail.

nix/host-modules/lib/hive-ca-trust.nix

@@ -170,13 +170,53 @@ in
               set -euo pipefail
               install -d -m 0755 ${dir}
               tmp=${bundlePath}.tmp
-              cat /etc/ssl/certs/ca-certificates.crt ${source} > "$tmp"
-              # `cat` of an empty or missing-but-mounted source exits 0, so the
-              # result has to be inspected rather than the command trusted.
-              if ! grep -q 'BEGIN CERTIFICATE' "$tmp"; then
-                echo "${unit}: assembled bundle contains no certificate" >&2
+              # Count the HIVE half on its own, before assembling.
+              #
+              # Inspecting the assembled file cannot work: the system store
+              # always holds certificates, so "does the result contain a
+              # certificate" passes unconditionally — including in the one case
+              # worth catching, where this source contributed nothing. And the
+              # public CAs are irrelevant to what this bundle is for: every name
+              # the consumer verifies is issued by our own CA, so a bundle of
+              # nothing but public CAs is, for this purpose, an empty one that
+              # measures as full.
+              #
+              # One helper, because the safe form is not the obvious one and
+              # three copies of a subtle idiom is three chances to get it wrong.
+              #
+              # `grep -c` has two different zero-ish outcomes and `set -e` is
+              # on: an EMPTY file prints `0` and exits 1, a MISSING file prints
+              # nothing and exits 2. So `|| true` leaves the variable empty on
+              # the second — the `-eq` then dies with "integer expected"
+              # instead of reporting — and `|| echo 0` appends a second zero on
+              # the first, yielding `00`. Normalising afterwards is the only
+              # form that survives both.
+              certs() {
+                n=$(grep -c 'BEGIN CERTIFICATE' "$1" 2>/dev/null || true)
+                if [ -z "$n" ]; then n=0; fi
+                printf '%s' "$n"
+              }
+              contributed=$(certs ${source})
+              if [ "$contributed" -eq 0 ]; then
+                echo "${unit}: ${source} contributed no certificate to the bundle" >&2
                 exit 1
               fi
+              system=$(certs /etc/ssl/certs/ca-certificates.crt)
+              cat /etc/ssl/certs/ca-certificates.crt ${source} > "$tmp"
+              # `cat` of a source truncated between the count and the copy still
+              # exits 0, so the result is checked against what both halves
+              # brought rather than merely for being non-empty.
+              total=$(certs "$tmp")
+              if [ "$total" -ne $((system + contributed)) ]; then
+                echo "${unit}: bundle has $total certificates, expected $system + $contributed" >&2
+                exit 1
+              fi
+              # ⚠️ Scope: this proves the anchor was CONTRIBUTED, not that it is
+              # USABLE. A consumer that reads only the first certificate of the
+              # file ignores it anyway — that is what took the swarm collector
+              # down — and no check on this file can see it. Only a real
+              # handshake can. Do not read a green assembly as a working trust
+              # store.
               chmod 0644 "$tmp"
               mv "$tmp" ${bundlePath}
             '';