Add example games and quick-start guide

Two example .ne programs with compiled .nes ROMs:
- hello_sprite: d-pad controlled sprite movement
- bouncing_ball: auto-bouncing sprite with edge detection

Includes README with build instructions and emulator setup.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

examples/bouncing_ball.ne

@@ -0,0 +1,37 @@
+// Bouncing Ball — a sprite that bounces around the screen automatically.
+//
+// Build:  cargo run -- build examples/bouncing_ball.ne
+// Output: examples/bouncing_ball.nes
+
+game "Bouncing Ball" {
+    mapper: NROM
+}
+
+var px: u8 = 64
+var py: u8 = 64
+var dx: u8 = 1     // 1 = moving right, 0 = moving left
+var dy: u8 = 1     // 1 = moving down,  0 = moving up
+
+on frame {
+    // Move horizontally
+    if dx == 1 {
+        px += 1
+        if px >= 240 { dx = 0 }
+    } else {
+        px -= 1
+        if px == 0 { dx = 1 }
+    }
+
+    // Move vertically
+    if dy == 1 {
+        py += 1
+        if py >= 224 { dy = 0 }
+    } else {
+        py -= 1
+        if py == 0 { dy = 1 }
+    }
+
+    draw Ball at: (px, py)
+}
+
+start Main