fix(alerting): evaluate the replay-paused branch of DbReplicationLag, add ApiMemoryPressure; retarget grey-zone scenarios

DbReplicationLag の式は PromQL の or が左辺のサンプル（常に 0）を残すため
pg_is_wal_replay_paused の分岐が評価されず、第 1・2 回とも発火しなかった。2 つの
bool を加算して clamp する式に直す。api コンテナのメモリが limit の 80% を超えて
2 分続いたときの ApiMemoryPressure（第 2 層）を追加する。C-G2 の StressChaos は
size "80%"（ノード基準）で即 OOM になっていたため 200MB に、V-G1 の対象は
トラフィックを受けているインスタンス legacy:busy に変更する。slo.md に新ルールを追記。

vm/harness/scenarios/C-G2.yaml

@@ -5,7 +5,7 @@ asset: container
 expected_layer: 2
 ground_truth_label: memory_pressure_api
 ground_truth_action: k8s.rollout_restart
-description: StressChaos memory workers 2 size 80%, app=api, duration 10m
+description: StressChaos memory workers 2 size 200MB (api limit 256Mi), app=api, duration 10m
 notes:
   open_item: "design 11.3: whether the stress re-attaches to the restarted pods for the remaining duration"
 inject:
@@ -24,7 +24,10 @@ inject:
       stressors:
         memory:
           workers: 2
-          size: "80%"
+          # 2026-10-08: "80%" is a share of the node memory (memStress reached 4.4 GB), so the cgroup OOM killer took memStress
+          # and the api process within a second: one restart per pod, no sustained pressure, no alert. 200MB stays under the
+          # 256Mi limit and keeps the working set near 90% for the whole duration (ApiMemoryPressure fires after 2m).
+          size: "200MB"
       duration: "10m"
 revert:
   method: chaos_delete

vm/harness/scenarios/V-G1.yaml

@@ -5,12 +5,15 @@ asset: vm
 expected_layer: 2
 ground_truth_label: network_latency_legacy
 ground_truth_action: mig.recreate_instance
-description: tc qdisc add dev ens4 root netem delay 400ms on legacy instance 0
+description: tc qdisc add dev ens4 root netem delay 400ms on the legacy instance that currently serves the api traffic (legacy:busy)
+# 2026-10-08: the ILB hashes the api pods' few keep-alive connections onto one instance; in campaign 2 legacy:0 served 0 rps,
+# so the delay was never on the request path. legacy:busy = the instance with the most legacy-api requests over the last
+# 2 minutes (Mimir); the harness caches the choice per scenario so the revert hits the same instance.
 inject:
   method: ssh
-  host: legacy:0
+  host: legacy:busy
   command: sudo tc qdisc add dev ens4 root netem delay 400ms
 revert:
   method: ssh
-  host: legacy:0
+  host: legacy:busy
   command: sudo tc qdisc del dev ens4 root || true