fix(deploy): relay firewall DROP must be appended, not -I-inserted above its ACCEPTs

iptables -I prepends, so the install block's final -I of the blanket DROP
landed it at position 1, above all three ACCEPTs: every packet to
18093/tcp died there (live 2026-10-21: 159 packets on the DROP, 0 on each
ACCEPT). The host's own health probe timed out beside a LISTENing socket
and the whole dankclaw drain aborted at the alertmanager gate twice.

Install now rebuilds the tail: delete any blanket DROP wherever it sits,
keep the ACCEPTs at the head, append the DROP last - converges on hosts
poisoned by the old order. New bats case runs install_signal_relay for
real against an iptables sandbox seeded with the poisoned order (red on
the old code, green on this one).

tests/observability_quadlet.bats

@@ -559,6 +559,64 @@ assert any('ALERTS' in e for e in exprs), exprs
     grep -q -- '--dport 18093 -j DROP' "$handler"
 }
 
+@test "the deployer installs the relay firewall with the ACCEPTs deciding before the DROP" {
+    # Behavioral, run for real against a sandbox that plays the kernel: the
+    # rule set is a LIST and iptables -I prepends, so the shape this function
+    # had until 2026-10-21 (-C/-I ACCEPT x3, then -C/-I the blanket DROP)
+    # landed the DROP at position 1 and every packet died there — the drain
+    # failed its health gate beside a LISTENing socket twice. Seeding the
+    # sandbox with that poisoned order also pins the convergent repair: a
+    # re-deploy must rewrite the tail, not skip because -C finds every rule.
+    local sandbox="$BATS_TEST_TMPDIR/fw"
+    mkdir -p "$sandbox/bin"
+    cat > "$sandbox/bin/iptables" <<'EOF'
+#!/usr/bin/env bash
+# ordered rule list in $FW_STATE: -C lookup, -I at head, -A at tail, -D delete.
+state="${FW_STATE:?}"
+touch "$state"
+op="$1"; shift
+case "$op" in
+    -C) grep -Fxq -- "$*" "$state" ;;
+    -I) grep -Fxq -- "$*" "$state" || { { echo "$*"; cat "$state"; } > "$state.new"; mv "$state.new" "$state"; } ;;
+    -A) grep -Fxq -- "$*" "$state" || echo "$*" >> "$state" ;;
+    -D) if grep -Fxq -- "$*" "$state"; then
+            grep -Fxv -- "$*" "$state" > "$state.new"; mv "$state.new" "$state"
+        else exit 1; fi ;;
+    *) exit 0 ;;
+esac
+EOF
+    chmod +x "$sandbox/bin/iptables"
+    # Poisoned state exactly as the pre-fix deployer left Dankclaw.
+    printf '%s\n' \
+        'INPUT -p tcp --dport 18093 -j DROP' \
+        'INPUT -p tcp --dport 18093 -s 10.89.0.0/16 -j ACCEPT' \
+        'INPUT -p tcp --dport 18093 -s 192.168.0.143 -j ACCEPT' \
+        'INPUT -p tcp --dport 18093 -s 127.0.0.1 -j ACCEPT' \
+        > "$sandbox/rules"
+    (
+        cd "$REPO_ROOT"
+        export PATH="$sandbox/bin:$PATH" FW_STATE="$sandbox/rules" HOME="$sandbox/home"
+        mkdir -p "$HOME"
+        eval "$(sed -n '/^install_signal_relay()/,/^}/p' scripts/deploy-remote.sh)"
+        run_privileged() { "$@"; }
+        systemctl() { :; }
+        curl() { return 0; }
+        sleep() { :; }
+        install_signal_relay
+    )
+    # Exactly one blanket DROP, and it sits after every ACCEPT.
+    [ "$(grep -cF -- 'INPUT -p tcp --dport 18093 -j DROP' "$sandbox/rules")" -eq 1 ]
+    local drop_at total
+    drop_at="$(grep -nF -- 'INPUT -p tcp --dport 18093 -j DROP' "$sandbox/rules" | cut -d: -f1)"
+    total="$(wc -l < "$sandbox/rules")"
+    [ "$drop_at" -eq "$total" ]
+    local s seen
+    for s in 127.0.0.1 192.168.0.143 10.89.0.0/16; do
+        seen="$(grep -nF -- "-s $s -j ACCEPT" "$sandbox/rules" | cut -d: -f1)"
+        [ -n "$seen" ] && [ "$seen" -lt "$drop_at" ]
+    done
+}
+
 @test "the planner selects alertmanager for its config and relay files" {
     run python3 "$REPO_ROOT/scripts/deploy-plan.py" --target dankclaw \
         --paths shared/prometheus/alertmanager.yml