refactor(game): lock-free storage, remove /command, flatten engine wrapper

Three-stage refactor of the game-engine plumbing (game logic untouched):

Stage 1 — lock-free persistence + admin serialisation. Remove the file
lock from repo/fs (the .lock file, the Read/Write-vs-*Safe duality and the
dead ReadSafe polling) and replace the two-step rename with a single atomic
rename so concurrent reads are torn-free without a lock. Serialise the
state-mutating admin writers (init/turn/banish) with one shared router
LimitMiddleware, rewritten to block on the request context instead of a
racy shared 100ms timer.

Stage 2 — remove the obsolete immediate-command path end to end. Players
submit through PUT /api/v1/order; the legacy PUT /api/v1/command path is
deleted across game (route, handler, 24 command factories, Ctrl), backend
(Commands handler/route, engineclient.ExecuteCommands), gateway (dispatch +
executeUserGamesCommand + routing entry), the FlatBuffers/model contract
(UserGamesCommand[Response]) and transcoder, plus every affected
OpenAPI/README/FUNCTIONAL/ARCHITECTURE doc. The integration proxy test is
converted to the order path.

Stage 3 — flatten the REST->engine wrapper. Replace the executor adapter,
the controller package functions and RepoController with one concrete
controller.Service; drop the single-implementation Repo and Storage
interfaces (repo.Repo / fs.FS are now concrete). Handlers depend on a thin
handler.Engine seam and own the domain->REST projection; storage is
resolved once at startup instead of per request.

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

pkg/model/rest/command.go

@@ -4,13 +4,10 @@ import "encoding/json"
 
 type Command struct {
 	Actor    string            `json:"actor" binding:"notblank"`
-	// Commands carries the engine-bound payload for either the
-	// command (`PUT /api/v1/command`, immediate) or the order
-	// (`PUT /api/v1/order`, validate-and-store) path. The order
-	// path treats an empty array as "the player has no orders for
-	// this turn" and stores it. The command handler still rejects
-	// an empty array by hand because immediate execution of a
-	// no-op makes no sense.
+	// Commands carries the engine-bound payload for the order
+	// (`PUT /api/v1/order`, validate-and-store) path. An empty array
+	// means "the player has no orders for this turn" and is stored
+	// as-is.
 	Commands []json.RawMessage `json:"cmd"`
 }