IR codegen: scroll, debug.log, debug.assert

Adds Scroll / DebugLog / DebugAssert variants to IrOp, wires them into
the IR lowering, optimizer liveness tracking, and IR codegen. Scroll
emits two PPU $2005 writes; debug statements emit writes to $4800 when
IrCodeGen::with_debug(true) is set, stripped otherwise. These were the
last feature gaps versus the AST codegen path, so IR codegen is now a
full replacement.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/main.rs

@@ -140,6 +140,7 @@ fn compile(input: &PathBuf, debug: bool, asm_dump: bool, use_ir: bool) -> Result
         use nescript::codegen::IrCodeGen;
         IrCodeGen::new(&analysis.var_allocations, &ir_program)
             .with_sprites(&sprites)
+            .with_debug(debug)
             .generate(&ir_program)
     } else {
         CodeGen::new(&analysis.var_allocations, &program.constants)