fix: Run mode behaviour (loop_once / loop_ntimes)

[mitchmindtree]

Summary

Reintroduces the pre-0.20 LoopMode::loop_once() behaviour that was lost in the Bevy port, and cleans up the surrounding RunMode API.

Previously, set_update_mode(UpdateMode::reactive(..)) keeps re-running on mouse movement, and no single UpdateMode can both ignore input and stay responsive, because bevy_winit cannot distinguish cursor movement from window close/resize (both are "window events").

What this adds

• RunMode::loop_once() and RunMode::loop_ntimes(n), with matching .loop_once() / .loop_ntimes(n) shortcuts on both the app Builder and the SketchBuilder. update and view each run exactly n times (default 1), then the last frame is held on screen while the window idles and stays closable and resizable. This is the intended mode for static, sketch-based compositions.
• App::set_run_mode(..) to change the run mode at runtime, mirroring App::set_update_mode. Loop modes can be entered and re-entered live.
• Two examples under examples/nannou_basics/:
  • loop_once - draws a static spiral once and holds it, and
  • run_modes - switches between Continuous, reactive rate, wait, and the loop modes at runtime with the keyboard.

How the frame is held

Simply capping view presents a black window. Under Bevy the drawn frame lives in the camera's persistent intermediate render texture, which is only kept populated while the window keeps drawing. Once drawing stops, empty passes plus MSAA-writeback ping-pong plus TextureCache eviction leave it black, so unfortunately our old approach of just re-using the texture is blocked at the moment.

Instead, in this workaround we freeze the Draw instance itself. A new nannou_draw::DrawFrozen resource gates the per-frame draw systems (reset_draw, clear_previous_frame, update_draw_mesh), so the last frame's rendered meshes and clear color persist and the render graph keeps drawing them. update/view are then skipped too, so they run exactly n times even for a sketch whose view reads live time or input. A Last-schedule system (apply_loop_once) counts rendered frames and, once the budget is spent, freezes the draw and switches to freeze() to idle. It runs in Last so the final frame's meshes, spawned in PostUpdate, survive into the next frame.

Breaking change: RunMode trimmed

RunMode is reduced to UntilExit and LoopNTimes. The Ticks, Duration and once() variants (run N frames or a duration, then quit) are removed. They were unused and undocumented, and once() (quit after one frame) was easily confused with loop_once() (hold after one frame). To run a fixed number of frames and then quit, call App::quit() from your own update once a counter reaches the target.

Migrating from the pre-0.20 LoopMode

• LoopMode::loop_once() -> RunMode::loop_once() (or .loop_once() on the app or sketch builder).
• LoopMode::loop_ntimes(n) -> RunMode::loop_ntimes(n).
• LoopMode::Wait -> app.set_update_mode(UpdateMode::wait()).
• LoopMode::Rate / rate_fps(fps) -> app.set_update_rate(fps) or UpdateMode::rate(hz).
• LoopMode::RefreshSync -> UpdateMode::Continuous (the default).