feat: DX runSimple, RunOptions, key helpers, 00_minimal

[debuggerdragon311]

• widget.key expported from widget root with isQuit, isUp, isDown, isLeft, isRight helpers — removes per-app boilerplate navigation functions
• RunOptions struct (alt_screen, fps, mouse, hide_cursor) — terminal behaviour configurable without manual escape sequences in main()
• runImpl extracted, runOpts added, run() kept as compat shim — no existing callers break
• runSimple — handles arena, process exit, and terminal cleanup. main() becomes 2 lines
• examples/00_minimal — demonstrates the new floor

Type of change

• New featuri
• Docs or examples only

Checklist

• zig build test passes on my machine
• zig fmt src/ examples/ build.zig has been run
• New public functions have doc comments
• No new cycle in the module dependency graph
• I can explain every line in this diff, including anything a model wrote

Notes for reviewers
run() is now a shim forwarding to runImpl with RunOptions{}, all defaults match previous hardcoded behaviour so existing apps compile unchanged. runOpts exists for callers who manage their own allocator. runSimple is the recommended entry point for new apps going forward. The sys.write newline in runSimple before exit(0) prevents the shell % partial-line indicator..