Add player-centered viewport scrolling for variable map sizes

Window size is now fixed (840×524) and independent of map dimensions.
Small maps center in the viewport; maps larger than the viewport keep
the player centered and clamp at board edges so no empty space shows.
Also derives Copy+Clone on Player to enable the per-frame origin calculation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/game.rs

@@ -190,6 +190,7 @@ pub struct PortalDef {
 /// the player will eventually become a scripted object that responds to
 /// input events, at which point this struct may be removed or made optional.
 /// See `ARCHITECTURE.md` for details.
+#[derive(Copy, Clone)]
 pub struct Player {
     /// Column position (0-indexed, increasing rightward).
     pub x: i32,