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>

game/internal/controller/order.go

@@ -127,7 +127,7 @@ func (c *Controller) applyOrders(t uint) error {
 	cmdApplied := make(map[string]bool)
 
 	for ri := range c.Cache.listRaceActingIdx() {
-		o, ok, err := c.Repo.LoadOrder(t, c.Cache.g.Race[ri].ID)
+		o, ok, err := c.repo.LoadOrder(t, c.Cache.g.Race[ri].ID)
 		if err != nil {
 			return err
 		}
@@ -166,7 +166,7 @@ func (c *Controller) applyOrders(t uint) error {
 			_ = c.applyCommand(commandRace[cmd.CommandID()], cmd)
 		}
 		// re-save order to persist possible changed commands result outcome
-		if err := c.Repo.SaveOrder(t, c.Cache.g.Race[ri].ID, &order.UserGamesOrder{
+		if err := c.repo.SaveOrder(t, c.Cache.g.Race[ri].ID, &order.UserGamesOrder{
 			GameID:    c.Cache.g.ID,
 			UpdatedAt: raceOrderUpdated[ri],
 			Commands:  raceOrder[ri],