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/ir/mod.rs

@@ -129,6 +129,14 @@ pub enum IrOp {
     ReadInput(IrTemp, u8),
     WaitFrame,
     Transition(String),
+    /// Write PPU scroll registers (two writes to $2005: X then Y).
+    Scroll(IrTemp, IrTemp),
+    /// Debug: write a list of temps to the emulator debug port ($4800).
+    /// Stripped in release mode by the codegen.
+    DebugLog(Vec<IrTemp>),
+    /// Debug: runtime assertion — if `cond` is zero, halt with debug marker.
+    /// Stripped in release mode by the codegen.
+    DebugAssert(IrTemp),
 
     // Source mapping
     SourceLoc(Span),