IR codegen: state dispatch, multi-OAM, and transition support

State machine dispatch:
- IrProgram now stores states (Vec<String>) and start_state
- Lowering captures state metadata before walking the AST
- IR codegen generates a main loop with vblank wait and CMP+BNE+JMP
  dispatch table, matching the AST codegen's layout
- Each frame handler ends with JMP __ir_main_loop
- current_state initialized to the start state's index at boot

Multi-OAM support:
- next_oam_slot counter, reset at the start of each _frame function
- Sequential allocation of 4-byte OAM entries at $0200 + slot*4
- Silently drops draws beyond slot 63 (OAM full)

Transition codegen:
- IrOp::Transition now looks up the target state's index from
  state_indices, writes it to ZP $03, and JMPs back to main loop
- Previously this was a no-op placeholder

Shared constants:
- ZP_FRAME_FLAG ($00) and ZP_CURRENT_STATE ($03) match AST codegen

Tests: 271 total (5 new IR codegen tests + 2 new integration tests)
All 7 examples compile through --use-ir, including multi-state games
and programs with transitions and functions.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/ir/mod.rs

@@ -33,6 +33,10 @@ pub struct IrProgram {
     pub functions: Vec<IrFunction>,
     pub globals: Vec<IrGlobal>,
     pub rom_data: Vec<IrRomBlock>,
+    /// Ordered list of state names (index = state dispatch number).
+    pub states: Vec<String>,
+    /// Name of the initial state when the ROM boots.
+    pub start_state: String,
 }
 
 /// A global variable in the IR.