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/hello_sprite.ne

@@ -0,0 +1,23 @@
+// Hello Sprite — the simplest NEScript program.
+//
+// Displays a smiley-face sprite on screen and moves it with the d-pad.
+// Build:  cargo run -- build examples/hello_sprite.ne
+// Output: examples/hello_sprite.nes (open in any NES emulator)
+
+game "Hello Sprite" {
+    mapper: NROM
+}
+
+var px: u8 = 128
+var py: u8 = 120
+
+on frame {
+    if button.right { px += 2 }
+    if button.left  { px -= 2 }
+    if button.down  { py += 2 }
+    if button.up    { py -= 2 }
+
+    draw Smiley at: (px, py)
+}
+
+start Main