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/parser/ast.rs

@@ -218,9 +218,7 @@ pub enum UnaryOp {
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum Player {
-    #[allow(dead_code)]
     P1,
-    #[allow(dead_code)]
     P2,
 }