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

examples/hello_sprite.ne

@@ -1,8 +1,12 @@
 // Hello Sprite — the simplest NEScript program.
 //
-// Displays a smiley-face sprite on screen and moves it with the d-pad.
+// Displays a 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)
+//
+// Note: In M1 the sprite name in `draw` is parsed but not resolved.
+// All sprites use tile 0 from the built-in CHR data (a smiley face).
+// Sprite declarations with custom tile data come in M3.
 
 game "Hello Sprite" {
     mapper: NROM