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/generate_turn.go

@@ -67,7 +67,7 @@ func (c *Controller) MakeTurn() error {
 	// Store bombings
 	bombingReport := make([]*report.Bombing, len(bombings))
 	if len(bombings) > 0 {
-		if err := c.Repo.SaveBombings(c.Cache.g.Turn, bombings); err != nil {
+		if err := c.repo.SaveBombings(c.Cache.g.Turn, bombings); err != nil {
 			return err
 		}
 		for i := range bombings {
@@ -107,7 +107,7 @@ func (c *Controller) MakeTurn() error {
 			}
 
 			report := TransformBattle(c.Cache, b)
-			if err := c.Repo.SaveBattle(c.Cache.g.Turn, report, &battleMeta[i]); err != nil {
+			if err := c.repo.SaveBattle(c.Cache.g.Turn, report, &battleMeta[i]); err != nil {
 				return err
 			}
 			battleReport[i] = report
@@ -118,12 +118,12 @@ func (c *Controller) MakeTurn() error {
 	c.Cache.DeleteKilledShipGroups()
 
 	// Store game state for the new turn and 'current' state as well
-	if err := c.Repo.SaveNewTurn(c.Cache.g.Turn, c.Cache.g); err != nil {
+	if err := c.repo.SaveNewTurn(c.Cache.g.Turn, c.Cache.g); err != nil {
 		return err
 	}
 
 	for rep := range c.Cache.Report(c.Cache.g.Turn, battleReport, bombingReport) {
-		if err := c.Repo.SaveReport(c.Cache.g.Turn, rep); err != nil {
+		if err := c.repo.SaveReport(c.Cache.g.Turn, rep); err != nil {
 			return err
 		}
 	}