Sprite resolution, asset wiring, shift-assign, unreachable state warning

Sprite/asset pipeline:
- Linker::link_with_assets() places sprite CHR data in ROM at correct tile
- assets::resolve_sprites() walks Program for inline sprite bytes
- CodeGen::with_sprites() maps sprite names to tile indices
- gen_draw() uses correct tile index from sprite declarations
- main.rs wires the full resolution pipeline

Shift-assign operators (<<= and >>=):
- AssignOp::ShiftLeftAssign and ShiftRightAssign variants
- Parser handles in both statement and array index contexts
- Codegen emits ASL A / LSR A
- IR lowering maps to ShiftLeft/ShiftRight ops

Unreachable state warning (W0104):
- BFS from start state finds reachable states via transitions
- States not reached produce W0104 warning

Error polish helpers:
- suggest_var_name() for "did you mean" suggestions
- emit_undefined_var() for E0502 with typo hints
- Used by analyzer for better diagnostics

242 tests pass, clippy clean.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/assets/mod.rs

@@ -1,7 +1,9 @@
 mod chr;
 mod palette;
+pub mod resolve;
 #[cfg(test)]
 mod tests;
 
 pub use chr::png_to_chr;
 pub use palette::{nearest_nes_color, NES_COLORS};
+pub use resolve::resolve_sprites;