feat(tls): adopt the swarm CA automatically, but only where it is safe

A hive whose CA predates the swarm root keeps a self-signed CA, and
nothing re-roots it. That is what makes the hierarchy non-disruptive,
and it also means the O(1)-trust payoff never arrives for that hive.

Adoption now happens by itself where this host owns the root, and
nowhere else. The split is the whole design: adoption invalidates an
anchor consumers already trust, and they refresh on their own schedule —
on one box that schedule is knowable, across hosts it is not. So the
all-local case migrates itself and every other case gets a failure that
names both files, gives the two-command recipe, says why it is not
automatic, and offers the marker as a deliberate opt-out.

The previous CA rides in the trust bundle afterwards. Consumers read the
bundle rather than ca.pem, so adoption is additive before it is
subtractive; agents pick up new trust only when their container
restarts, which is a window even on a single host. Dropping the old
anchor stays a separate, deliberate step.

nix/host-modules/hive-tls.nix

@@ -248,6 +248,49 @@ in
         root=${lib.escapeShellArg "${swarmCaCfg.stateDir}/root.pem"}
         rootk=${lib.escapeShellArg "${swarmCaCfg.stateDir}/root-key.pem"}
 
+        prev="$d/ca-previous.pem"
+        marker="$d/.swarm-ca-adopted"
+
+        # --- Adoption: a hive whose CA predates the swarm root.
+        #
+        # Runs ONLY where this host also owns the root (`autoConfigure`),
+        # because adoption invalidates an anchor that consumers already
+        # trust and they refresh on their own schedule. On one box that
+        # schedule is knowable; across hosts it is not, so there the
+        # operator does it and this unit only says so, loudly.
+        #
+        # Guarded by a marker rather than by the state of the world: the
+        # marker's ABSENCE is the trigger, so this fires once per hive
+        # instead of re-deciding every activation. That is the difference
+        # between a migration and a kicking machine.
+        if [ -s "$root" ] && [ -s "$ca" ] && [ ! -e "$marker" ] \
+            && ! openssl verify -CAfile "$root" "$ca" >/dev/null 2>&1; then
+        ${
+          if swarmCaCfg.autoConfigure then
+            ''
+              echo "adopting the swarm CA: $ca does not chain to $root" >&2
+              # Keep the old CA as an anchor across the overlap. Consumers
+              # read the bundle, so adoption is additive before it is
+              # subtractive — agents pick up new trust only when their
+              # container restarts, which is a window even on one host.
+              cp "$ca" "$prev"
+              chmod 0644 "$prev"
+              rm -f "$ca" "$cak"
+              touch "$marker"''
+          else
+            ''
+              echo "hive-tls: this hive's CA does not chain to the swarm root." >&2
+              echo "  hive CA:    $ca" >&2
+              echo "  swarm root: $root" >&2
+              echo "Adopting the hierarchy is not automatic here: it invalidates an" >&2
+              echo "anchor that peers and agents on OTHER hosts still trust, and they" >&2
+              echo "refresh on their own schedule — only you know when that is safe." >&2
+              echo "To adopt: rm $ca $cak && systemctl restart hive-tls-ca.service" >&2
+              echo "To keep the current CA deliberately: touch $marker" >&2
+              exit 1''
+        }
+        fi
+
         # --- CA: generated once and reused across leaf rotations, in
         # whichever of the two shapes `caGenScript` selected.
         # Regenerated only if missing or already expired (checkend 0).
@@ -298,12 +341,24 @@ in
         # Written IN PLACE, never renamed into position: containers bind
         # -mount this file, and a bind mount follows the inode. A
         # rename would leave every consumer holding the old one.
+        # `ca-previous.pem` rides along after an adoption for the same
+        # reason: a consumer that has not restarted yet is still pinned to
+        # the old anchor, and dropping it the instant the new one exists
+        # would break exactly the hosts the migration was supposed to
+        # spare. Removing it is a deliberate later step, not this unit's
+        # call — "long enough" is a deployment fact.
+        #
+        # Built as an explicit anchor ARRAY with `if` guards, not
+        # `[ -s f ] && anchors+=(f)`: under `set -e` an && list whose test
+        # fails IS a failing command and kills the unit, so the tidier
+        # form would break precisely on the hive where the optional file
+        # is legitimately absent. The `|| true` patch for that is worse —
+        # it would also swallow a real failure to read the hive CA.
         bundle="$d/trust-bundle.pem"
-        if [ -s "$root" ]; then
-          cat "$ca" "$root" > "$bundle"
-        else
-          cat "$ca" > "$bundle"
-        fi
+        anchors=("$ca")
+        if [ -s "$prev" ]; then anchors+=("$prev"); fi
+        if [ -s "$root" ]; then anchors+=("$root"); fi
+        cat "''${anchors[@]}" > "$bundle"
         chmod 0644 "$bundle"
       '';
     };