feat(kv): set_if compare-and-swap for KV (per-app + shared + SDK) (Track A M5)

No atomic CAS existed, so concurrent writers to the same KV key raced. Add
set_if(key, expected, new): writes only when the current value equals expected,
or (expected absent) only when the key is missing — the primitive for lock-free
counters / optimistic concurrency.

- KvService + GroupKvService traits gain set_if with a defaulted "unsupported"
  impl (Noop + other impls untouched); the real services override it.
- kv_repo/group_kv_repo: atomic set_if — a single UPDATE ... WHERE value =
  $expected (JSONB semantic equality) or INSERT ... ON CONFLICT DO NOTHING.
  Atomic without a transaction; returns whether the write happened.
- Services enforce the same value-size cap + (group) writes-authed + row/byte
  quotas as set; the event fires only on a successful swap.
- Executor Rhai SDK: kv::collection(c).set_if(key, expected, new) and the
  shared_collection variant; Rhai () for expected means "only if absent".

Pinned by kv_service/group_kv_service set_if unit tests (CAS + fail-closed) +
sdk_kv::kv_set_if_compare_and_swap (through a real script). No migration.
docs/sdk-shape.md documents the primitive.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/kv_service.rs

@@ -186,6 +186,59 @@ impl KvService for KvServiceImpl {
         Ok(())
     }
 
+    async fn set_if(
+        &self,
+        cx: &SdkCallCx,
+        collection: &str,
+        key: &str,
+        expected: Option<serde_json::Value>,
+        new: serde_json::Value,
+    ) -> Result<bool, KvError> {
+        validate_collection(collection)?;
+        let encoded_len = serde_json::to_vec(&new)
+            .map(|v| v.len())
+            .map_err(|e| KvError::Backend(format!("encode value: {e}")))?;
+        if encoded_len > self.max_value_bytes {
+            return Err(KvError::ValueTooLarge {
+                limit: self.max_value_bytes,
+                actual: encoded_len,
+            });
+        }
+        self.check_write(cx).await?;
+        // `insert` when the precondition was "absent", else `update`. The op is
+        // only emitted on a successful swap.
+        let op = if expected.is_some() {
+            "update"
+        } else {
+            "insert"
+        };
+        let swapped = self
+            .repo
+            .set_if(cx.app_id, collection, key, expected, new.clone())
+            .await?;
+        if swapped {
+            // Non-transactional with the write (same caveat as `set`).
+            if let Err(e) = self
+                .events
+                .emit(
+                    cx,
+                    ServiceEvent {
+                        source: "kv",
+                        op,
+                        collection: Some(collection.to_string()),
+                        key: Some(key.to_string()),
+                        payload: Some(new),
+                        old_payload: None,
+                    },
+                )
+                .await
+            {
+                tracing::error!(error = %e, source = "kv", op, event_emit_failure = true, "event emit failed");
+            }
+        }
+        Ok(swapped)
+    }
+
     async fn delete(&self, cx: &SdkCallCx, collection: &str, key: &str) -> Result<bool, KvError> {
         validate_collection(collection)?;
         self.check_write(cx).await?;
@@ -283,6 +336,28 @@ mod tests {
                 .insert((app_id, collection.to_string(), key.to_string()), value))
         }
 
+        async fn set_if(
+            &self,
+            app_id: AppId,
+            collection: &str,
+            key: &str,
+            expected: Option<serde_json::Value>,
+            new: serde_json::Value,
+        ) -> Result<bool, KvRepoError> {
+            // Holding the map lock makes the compare-and-set atomic in-memory.
+            let mut data = self.data.lock().await;
+            let k = (app_id, collection.to_string(), key.to_string());
+            let matches = match (&expected, data.get(&k)) {
+                (None, None) => true,                 // insert-if-absent
+                (Some(exp), Some(cur)) => exp == cur, // swap-if-equal
+                _ => false,
+            };
+            if matches {
+                data.insert(k, new);
+            }
+            Ok(matches)
+        }
+
         async fn delete(
             &self,
             app_id: AppId,
@@ -464,6 +539,63 @@ mod tests {
         assert!(matches!(err, KvError::InvalidCollection));
     }
 
+    #[tokio::test]
+    async fn set_if_is_compare_and_swap() {
+        let kv = svc();
+        let cx = anon_cx(AppId::new());
+        // Insert-if-absent: expected () swaps only when the key is missing.
+        assert!(
+            kv.set_if(&cx, "cas", "k", None, serde_json::json!(1))
+                .await
+                .unwrap(),
+            "insert-if-absent must succeed on a missing key"
+        );
+        assert!(
+            !kv.set_if(&cx, "cas", "k", None, serde_json::json!(2))
+                .await
+                .unwrap(),
+            "insert-if-absent must fail once the key exists"
+        );
+        assert_eq!(
+            kv.get(&cx, "cas", "k").await.unwrap(),
+            Some(serde_json::json!(1))
+        );
+
+        // Swap-if-equal: wrong expected → no-op; correct expected → swaps.
+        assert!(
+            !kv.set_if(
+                &cx,
+                "cas",
+                "k",
+                Some(serde_json::json!(99)),
+                serde_json::json!(3)
+            )
+            .await
+            .unwrap(),
+            "a mismatched expected must not swap"
+        );
+        assert_eq!(
+            kv.get(&cx, "cas", "k").await.unwrap(),
+            Some(serde_json::json!(1))
+        );
+        assert!(
+            kv.set_if(
+                &cx,
+                "cas",
+                "k",
+                Some(serde_json::json!(1)),
+                serde_json::json!(3)
+            )
+            .await
+            .unwrap(),
+            "a matching expected must swap"
+        );
+        assert_eq!(
+            kv.get(&cx, "cas", "k").await.unwrap(),
+            Some(serde_json::json!(3))
+        );
+    }
+
     /// Load-bearing: a script with `cx.app_id = A` must NOT see
     /// entries inserted under `cx.app_id = B`. This is the cross-app
     /// isolation boundary; getting this wrong is a security