M2: Add function/array parsing, IR data structures and lowering

Parser extensions:
- Function declarations with params and return types (fun, inline fun)
- Array type syntax (u8[16]), array literal expressions ([1, 2, 3])
- fast/slow variable placement hints
- Functions stored in Program AST

IR module (new):
- IrProgram, IrFunction, IrBasicBlock, IrOp, IrTerminator types
- AST-to-IR lowering pass with:
  - Expression lowering to virtual temps
  - Control flow (if/else, while, loop) to basic blocks with branches
  - Break/continue via loop context stack
  - Short-circuit logical and/or
  - Button reads, draw sprites, wait_frame
  - Constants inlined as LoadImm
  - State handlers lowered as separate IR functions

Tests: 6 new parser tests, 11 new IR lowering tests (153 total)

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/codegen/mod.rs

@@ -395,7 +395,7 @@ impl CodeGen {
                 self.emit(LDA, AM::ZeroPage(self.input_addr));
                 self.emit(AND, AM::Immediate(mask));
             }
-            Expr::Call(_, _, _) | Expr::ArrayIndex(_, _, _) => {
+            Expr::Call(_, _, _) | Expr::ArrayIndex(_, _, _) | Expr::ArrayLiteral(_, _) => {
                 // TODO: implement for later milestones
             }
         }