Add example builds to CI, document sprite name behavior

- CI: new "Build Examples" job compiles all .ne files and validates
  the output ROMs have correct iNES headers
- Examples: add notes explaining that sprite names (Smiley, Ball) are
  parsed but not yet resolved — all draws use the built-in CHR tile 0.
  Custom sprite declarations come in M3.
- Codegen: explicit `let _ = &draw.sprite_name` to document the
  intentional skip, with comment about M2/M3 scope

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/codegen/mod.rs

@@ -540,7 +540,10 @@ impl CodeGen {
     fn gen_draw(&mut self, draw: &DrawStmt) {
         // OAM buffer is at $0200-$02FF
         // Each sprite entry: Y, tile, attributes, X
-        // For M1: use OAM slot 0 (bytes $0200-$0203)
+        // For M1: sprite name (draw.sprite_name) is parsed but ignored —
+        // all draws use OAM slot 0 with tile index 0 (the built-in CHR tile).
+        // Sprite name resolution and multiple OAM slots come in M2/M3.
+        let _ = &draw.sprite_name;
         // Y position (stored at $0200)
         self.gen_expr(&draw.y);
         self.emit(STA, AM::Absolute(0x0200));