Add Player 2 controller support

Parser:
- p1.button.X and p2.button.X syntax
- Emits Expr::ButtonRead with Some(Player::P1) or Some(Player::P2)
- Plain button.X still works (defaults to P1)
- Removed #[allow(dead_code)] from Player::P1 and Player::P2

Runtime:
- NMI handler now reads both $4016 (JOY1) and $4017 (JOY2) in
  the same loop, shifting 8 bits into ZP_INPUT_P1 ($01) and
  ZP_INPUT_P2 ($08) simultaneously
- JOY2 register constant added

Codegen:
- Button reads select correct ZP address based on Player variant
- gen_condition and gen_expr both handle P1/P2 dispatching

Tests: 245 (3 new parser tests: p1/p2 button read + shift-assign)

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/runtime/mod.rs

@@ -16,6 +16,7 @@ const APU_FRAME: u16 = 0x4017;
 /// Zero-page locations used by the runtime.
 pub const ZP_FRAME_FLAG: u8 = 0x00;
 pub const ZP_INPUT_P1: u8 = 0x01;
+pub const ZP_INPUT_P2: u8 = 0x08;
 
 /// Generate the NES hardware initialization sequence.
 /// This runs at RESET and sets up the hardware before user code.
@@ -110,12 +111,17 @@ pub fn gen_nmi() -> Vec<Instruction> {
     out.push(Instruction::new(LDA, AM::Immediate(0x00)));
     out.push(Instruction::new(STA, AM::Absolute(JOY1)));
 
-    // Read 8 button bits into ZP_INPUT_P1
+    // Read 8 button bits from controller 1 ($4016) into ZP_INPUT_P1
+    // and 8 button bits from controller 2 ($4017) into ZP_INPUT_P2
+    // simultaneously — shift each port's carry into its ZP byte.
     out.push(Instruction::new(LDX, AM::Immediate(0x08)));
     out.push(Instruction::new(NOP, AM::Label("__read_input".into())));
     out.push(Instruction::new(LDA, AM::Absolute(JOY1)));
     out.push(Instruction::new(LSR, AM::Accumulator));
     out.push(Instruction::new(ROL, AM::ZeroPage(ZP_INPUT_P1)));
+    out.push(Instruction::new(LDA, AM::Absolute(0x4017))); // JOY2
+    out.push(Instruction::new(LSR, AM::Accumulator));
+    out.push(Instruction::new(ROL, AM::ZeroPage(ZP_INPUT_P2)));
     out.push(Instruction::implied(DEX));
     out.push(Instruction::new(
         BNE,