feat(identity): generic game-scoped external-identity store

openfut-identity: durable, reversible (game_id, entity_kind, core_id) <->
external wire id mapping. Game-independent infrastructure (adapters supply the
numeric policy via base_floor; the store guarantees stable/unique/reversible/
game-scoped/persistent/atomic/explicit). JSON-file backed behind an
ExternalIdentityStore trait (SQLite can drop in later); parking_lot-guarded,
atomic temp+rename persist, rejects a torn reverse-duplicate on open.

Core never learns FIFA integers; only the host/adapter that owns a game
boundary uses this. 6 tests, 4/4 mutations killed (same-id-for-two-items,
lost-on-restart, broken-reverse, dropped-game-scope). Phase commit 1/5.

Cargo.lock

@@ -3202,6 +3202,16 @@ dependencies = [
 name = "openfut-http"
 version = "0.1.0"
 
+[[package]]
+name = "openfut-identity"
+version = "0.1.0"
+dependencies = [
+ "parking_lot",
+ "serde",
+ "serde_json",
+ "tempfile",
+]
+
 [[package]]
 name = "openfut-launcher"
 version = "0.1.0"