feat: OnClose, a handler for the window's close request

[LucasAntunesdeAlmeida]

What

g.OnClose(fn func()): run fn once when the player closes the window (X button, Alt+F4, the taskbar's Close, a launcher such as Steam asking the game to exit), then end the game like Quit. Until now a close request ended the game at once, with no chance to save progress or bank a run in progress.

Design choices

• Built on ebiten.SetWindowClosingHandled(true) + ebiten.IsWindowBeingClosed(). OnClose sets the flag (OnClose(nil) gives the close button back to the window); the runner checks for a close request at the start of Update.
• The handler runs on the game loop, between two frames, like any other callback, so it can read and save game state safely. Then the next frame never runs: Update returns ebiten.Termination, Run returns and saves the COLLIDER_RECORD GIF, the same path as Quit.
• Once: the handler is cleared before it runs.
• No veto. Launchers and the OS expect a close request to end the game; an "are you sure?" on close is not what a small API needs. A game that wants one can still offer its own QUIT entry.
• Not on Quit (the game's own decision), not headless (no window: Step and COLLIDER_AGENT=mcp never see it), and a no-op in a browser: a tab closes its page and ebiten reports no close request there, so games save as they go (localStorage writes are immediate). Autopilot and mcp-window are real windows: the handler runs.
• windowBeingClosed / setWindowClosingHandled are swappable variables so tests fake the window.

Proof

• close_test.go: the handler runs once, its Save lands, no frame runs after it, later Updates stay Termination; Quit does not run it; OnClose(nil) restores the default; headless never runs it.
• examples/catcher: closing the window mid-round keeps a new best (g.OnClose saves it); its README explains the pattern.
• Real window check on Windows: a probe program with OnClose writing a file, closed by WM_CLOSE (Process.CloseMainWindow): the handler ran after 135 frames, then Run returned and the process exited.
• README: cheat-sheet row, Catcher row, contract item 8.
• Local gate: gofmt, vet, js/wasm vet + build, staticcheck, go test.