replay: route player input through a command chokepoint (Phase 1)

Reshapes every UI-driven sim mutation to flow through one path so it can be
recorded and replayed later, with behaviour unchanged.

- Command model (lib): Command base + derived types (PlaceBuilding, Demolish,
  RotateInPlace, SetRecipe, SetShipLayout, Set[Site]SplitterFilters,
  ClearBeltTiles, ApplySchematicChoice, Reset), each with a playerId for the
  future-multiplayer shape. PlaceBuilding is atomic (carries optional
  recipe/layout/filters) because deferred commands never return the new
  BuildingId to the caller.
- CommandManager (lib): FIFO queue holding a Simulation&; enqueue + drain.
- Simulation::apply(const Command&): the single chokepoint, dispatching by
  kind to the existing mutators. Mutators stay public (enforced by convention,
  not compile-time, so the test suite keeps driving the sim directly).
- Timing: GameWorldView owns the CommandManager and drains it once per frame in
  onFrame, before the tick batch (runs at 0x too, so build-while-paused is
  preserved). A drained Reset triggers the view reset.
- UI fan-in: GameWorldView enqueues its own input directly; MainWindow and
  SelectedBuildingPanel emit CommandRequestedEvent, which GameWorldView
  subscribes to and enqueues. No UI site mutates the sim directly anymore.
- CommandTest: asserts apply(...) yields byte-identical state to the direct
  mutator path, and that CommandManager drains FIFO through apply.

Full suite green (338 cases / 3353 assertions); determinism double-run still
passes. Design doc updated with the atomic-PlaceBuilding and convention-
enforcement decisions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DUsFgd2Ga6pmLz8giS8WUn

src/lib/sim/Simulation.h

@@ -24,6 +24,7 @@
 
 class AiSystem;
 class BuildingSystem;
+struct Command;
 class Hasher;
 class CombatSystem;
 class DynamicBodySystem;
@@ -51,6 +52,12 @@ public:
     // Advances the simulation by one tick. Tick order per architecture.md §Tick Order.
     void tick();
 
+    // The single command chokepoint: applies one player command by dispatching
+    // to the underlying mutators. Every sim mutation during play must flow
+    // through here so it can be recorded and replayed (see docs/replay_design.md
+    // and CommandManager). Reached via CommandManager::drain.
+    void apply(const Command& command);
+
     // Returns all fire events accumulated since the last drain, clearing the
     // internal queue. Call once per rendered frame (REQ-SHP-FIRING-BEAM).
     std::vector<BeamFiredEvent> drainBeamFiredEvents();