`pyssg.watch`

Filesystem watch mode for pyssg.

Public surface:

:class:FsEvent and :func:coalesce -- the neutral event type and its pure debounce merge;
:class:FsWatcher -- the native, event-driven watcher;
:func:is_ignored -- the pure ignore-glob helper.

This package is the only place allowed to import watchdog; everything it exposes upward is backend-neutral.