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/executor-core/src/sdk/kv.rs

@@ -5,6 +5,8 @@
 //! widgets.set("k", #{ n: 1 });
 //! let v = widgets.get("k");      // value or () if absent
 //! if widgets.has("k") { ... }
+//! widgets.set_if("k", (), #{ n: 0 });   // insert only if ABSENT -> bool
+//! widgets.set_if("k", old, new);        // swap only if current == old -> bool
 //! widgets.delete("k");            // bool (was-present)
 //! let page = widgets.list();      // returns #{ keys: [...], next_cursor: () }
 //! ```
@@ -105,6 +107,7 @@ pub(super) fn register(engine: &mut RhaiEngine, services: &Services, cx: Arc<Sdk
 
     register_get(engine);
     register_set(engine);
+    register_set_if(engine);
     register_has(engine);
     register_delete(engine);
     register_list(engine);
@@ -114,11 +117,23 @@ pub(super) fn register(engine: &mut RhaiEngine, services: &Services, cx: Arc<Sdk
 
     register_group_get(engine);
     register_group_set(engine);
+    register_group_set_if(engine);
     register_group_has(engine);
     register_group_delete(engine);
     register_group_list(engine);
 }
 
+/// Map a Rhai `expected` argument to the CAS precondition: Rhai unit `()` means
+/// "expected ABSENT" (insert-if-absent); any other value is the expected current
+/// value.
+fn expected_from_dynamic(expected: &Dynamic) -> Option<serde_json::Value> {
+    if expected.is_unit() {
+        None
+    } else {
+        Some(dynamic_to_json(expected))
+    }
+}
+
 fn register_get(engine: &mut RhaiEngine) {
     engine.register_fn(
         "get",
@@ -145,6 +160,27 @@ fn register_set(engine: &mut RhaiEngine) {
     );
 }
 
+fn register_set_if(engine: &mut RhaiEngine) {
+    // `handle.set_if(key, expected, new)` — CAS. `expected = ()` means "only if
+    // absent". Returns `true` if the swap happened, `false` if the precondition
+    // failed.
+    engine.register_fn(
+        "set_if",
+        |handle: &mut KvHandle,
+         key: &str,
+         expected: Dynamic,
+         new: Dynamic|
+         -> Result<bool, Box<EvalAltResult>> {
+            let h = handle.clone();
+            let exp = expected_from_dynamic(&expected);
+            let new = dynamic_to_json(&new);
+            block_on("kv", async move {
+                h.service.set_if(&h.cx, &h.collection, key, exp, new).await
+            })
+        },
+    );
+}
+
 fn register_has(engine: &mut RhaiEngine) {
     engine.register_fn(
         "has",
@@ -243,6 +279,24 @@ fn register_group_set(engine: &mut RhaiEngine) {
     );
 }
 
+fn register_group_set_if(engine: &mut RhaiEngine) {
+    engine.register_fn(
+        "set_if",
+        |handle: &mut GroupKvHandle,
+         key: &str,
+         expected: Dynamic,
+         new: Dynamic|
+         -> Result<bool, Box<EvalAltResult>> {
+            let h = handle.clone();
+            let exp = expected_from_dynamic(&expected);
+            let new = dynamic_to_json(&new);
+            block_on("kv", async move {
+                h.service.set_if(&h.cx, &h.collection, key, exp, new).await
+            })
+        },
+    );
+}
+
 fn register_group_has(engine: &mut RhaiEngine) {
     engine.register_fn(
         "has",