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/error/generic.go

@@ -22,8 +22,7 @@
 //	    Game-state runtime rejections that depend on the current state
 //	    snapshot: entity-not-found, not-owned, in-use, ships-busy,
 //	    insufficient resources, send/upgrade/cargo dependencies. These
-//	    surface as per-command `cmdErrorCode` on PUT /api/v1/order
-//	    (and only escape as HTTP 400 from PUT /api/v1/command).
+//	    surface as per-command `cmdErrorCode` on PUT /api/v1/order.
 //
 // Code 0 represents "applied without error" and is reserved as the
 // successful per-command outcome on CommandMeta.Result. Code -1
@@ -115,8 +114,7 @@ func IsInputCode(code int) bool { return code >= 2000 && code < 3000 }
 
 // IsGameStateCode reports whether code belongs to the game-state /
 // per-command rejection shelf (3xxx). On PUT /api/v1/order these are
-// recorded into CommandMeta.CmdErrCode; on PUT /api/v1/command they
-// map to HTTP 400.
+// recorded into CommandMeta.CmdErrCode.
 func IsGameStateCode(code int) bool { return code >= 3000 && code < 4000 }
 
 func GenericErrorText(code int) string {