Embed the sim in the game binary; drop networking

The client becomes the game: it owns a Sim instance and advances it with a
fixed-timestep accumulator (24 Hz ticks, rendering at frame rate). Rendering,
pathfinding and collision read the World directly instead of a chunk cache.

Sim::pending (window slot maps, late rule, horizon) is replaced by one intent
per entity for the next movement window — set_action replaces, executing the
window consumes. The whole client prediction/reconciliation machinery
(tick estimation, RTT lead, retraction, replay) is gone with the latency it
was built for; click-to-move now syncs the route against the sim position
after every window.

Removed: server, netsim, shared (wire types), client/net.rs. client renamed
to game. Headless tests cover the intent model.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

sim/src/lib.rs

@@ -85,10 +85,6 @@ pub const TICK_HZ: u32 = 24;
 /// `TICKS_PER_MOVE` ticks (6 Hz). Window `w` executes at tick `w * TICKS_PER_MOVE`.
 pub const TICKS_PER_MOVE: u32 = 4;
 
-/// How many *future* movement windows an actor may address. Actions targeted beyond
-/// this horizon are dropped.
-pub const ACTION_WINDOW_HORIZON: usize = 3;
-
 /// Movement vocabulary: whether a tile id blocks movement. Keep this in lockstep with
 /// the tileset in `overworld.tga`.
 ///