fs: an event-driven file watch (inotify / FSEvents / ReadDirectoryChangesW) for hot reload #21

What

An event-driven file watch in mcpplibs.os::fs: a program says which files or directories to watch and is told when one changes, instead of polling last_write_time.

Why

openmui's development mode reloads an application's TypeScript, views and themes when they are saved (hot reload, live design). The target is save-to-screen ≤ 100 ms at p95. Today the player polls each watched file's modification time every frame (1/60 s) in development; that meets the budget but wakes the program every frame even when nothing changes, and the cost grows with the number of files (a project's imported modules, components, themes).

Shape (a suggestion)

auto w = mcpplibs::os::fs::watcher::open();          // result<watcher>
w->add(path);                                         // a file, or a directory (recursive optional)
auto changed = w->take();                             // non-blocking: the paths changed since the last take
auto changed = w->wait_for(std::chrono::milliseconds); // or wait
int fd = w->native_handle();                          // to join an event loop (where the platform has one)

Backends: inotify (Linux), FSEvents or kqueue (macOS), ReadDirectoryChangesW (Windows); errc::not_supported where there is none (Emscripten), so callers fall back to polling. Editors that save by rename (write a temporary, rename over) should still report the watched path.