Add controller 2 bindings for special game functions

[tmustier]

Context

Some NES games (e.g. Legend of Zelda) use controller 2 for special actions like save (Up + A on controller 2). We only map controller 1 today, so the save trick can’t be triggered without also moving/attacking on P1.

Goal

Add optional controller-2 keybindings so players can trigger special functions without interfering with P1 inputs.

Notes

• Input system already supports multi-key combos; this is about mapping to player 2.
• Need to choose a default P2 mapping that avoids accidental interference (e.g. use a right-hand cluster like IJKL + U/O, or use modifier-based mappings like Alt+WASD + Alt+Z/X).
• Should be configurable in ~/.pi/nes/config.json (e.g. keybindingsP2 or players: {1,2}), with docs updated.

Tasks

• Add controller-2 mapping support in the input handler (jsnes controller index 2).
• Decide on a default P2 keybinding scheme that minimizes accidental overlap.
• Add config schema + docs for P2 bindings.
• Validate Zelda save (Up + A on controller 2) works without interfering with P1.