feat: implement keyboard input function

[mjm918]

Summary

Add keyboard input capability to naml runtime to enable interactive programs.

Proposed API

// Blocking read - waits for user input
fn read_line() -> string;

// Non-blocking key read (for games) - returns key code or -1 if no key pressed
fn read_key() -> int;

Use Cases

• Interactive CLI applications
• Games (snake, tetris, etc.)
• User prompts and menus

Implementation Notes

• read_line() - blocking, reads until newline
• read_key() - non-blocking, requires terminal raw mode
• Platform considerations:
  • Native: use std::io::stdin() with raw mode via termios
  • Server WASM: WASI stdin
  • Browser: Not applicable (use DOM events instead)

Acceptance Criteria

• read_line() reads a line of text from stdin
• read_key() returns key code without blocking
• Arrow keys return distinct codes
• Works on native platform

[mjm918]

Implemented in commit 92b1f60. read_line() is a built-in, read_key() is in std::io.