feat(squad): generic game-scoped opaque extension + server fingerprint, atomic in replace_squad tx

src/services/game_ext.rs

@@ -0,0 +1,34 @@
+//! Generic read/write for [`crate::models::game_ext`] opaque state.
+//!
+//! Core never interprets the payload. Writes happen INSIDE the owning entity's
+//! transaction (see `squad::replace_squad_with_extension`) so the canonical
+//! entity and its opaque extension commit atomically — there is deliberately no
+//! standalone "write extension" entry point that could desync the two.
+
+use crate::db::Pool;
+use crate::error::AppResult;
+use crate::models::game_ext::GameEntityExt;
+
+/// Fetch the stored opaque extension for a scoped entity, or `None`. The caller
+/// compares `canonical_fingerprint` against the entity's *current* fingerprint to
+/// decide freshness — this layer does not know how to fingerprint any entity.
+pub async fn get_ext(
+    pool: &Pool,
+    game_id: &str,
+    entity_kind: &str,
+    entity_id: &str,
+    namespace: &str,
+) -> AppResult<Option<GameEntityExt>> {
+    let row = sqlx::query_as::<_, GameEntityExt>(
+        "SELECT game_id, entity_kind, entity_id, namespace, schema_version, \
+         canonical_fingerprint, payload, updated_at FROM game_entity_ext \
+         WHERE game_id = ? AND entity_kind = ? AND entity_id = ? AND namespace = ?",
+    )
+    .bind(game_id)
+    .bind(entity_kind)
+    .bind(entity_id)
+    .bind(namespace)
+    .fetch_optional(pool)
+    .await?;
+    Ok(row)
+}