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/router/healthz_test.go

@@ -6,7 +6,8 @@ import (
 	"net/http/httptest"
 	"testing"
 
-	"galaxy/game/internal/controller"
+	"galaxy/util"
+
 	"galaxy/game/internal/router"
 	"galaxy/game/internal/router/handler"
 
@@ -15,9 +16,10 @@ import (
 )
 
 func TestHealthzReturnsOKWithoutInit(t *testing.T) {
-	r := router.SetupRouter(handler.NewDefaultConfigExecutor(func(p *controller.Param) {
-		p.StoragePath = ""
-	}))
+	root, cleanup := util.CreateWorkDir(t)
+	defer cleanup()
+
+	r := router.SetupRouter(newService(t, root))
 
 	w := httptest.NewRecorder()
 	req, _ := http.NewRequest(http.MethodGet, "/healthz", nil)