Problem:
Directory listing entries cannot be customized (filtered, reordered).
Listings are read by a BufReadCmd, which suppresses BufReadPost, so they
are the only buffers with no post-read event to hook.

Solution:
Introduce a post-render User autocmd `DirReadPost`, marking the dir
buffer writable for the duration and before the cursor is placed, so
handlers can sort or filter it with ordinary commands. Document common
recipes

Problem:
`1-` does nothing from a directory buffer, because we are already in the
buffer-local CWD. It's also unintuitive that this mapping behaves
differently based on the resolved CWD.

Solution:
Have `1-` open the global CWD.

Problem:  An error raised after the list handler ran (e.g. while
          rendering) unwinds through the pcall around provider.list()
          and is routed back into the now no-op handler: silently
          discarded.
Solution: Re-raise the error when the handler has already run.

Problem:  A failed initial directory listing discards the listing state:
          the user is left in an empty modifiable non-listing buffer, and
          the BufEnter autocmd retries the failing open, repeating the
          error on every re-entry.
Solution: Render the failure as an empty listing with state and handlers
          intact: the error is reported once and "R" or :edit retries the
          listing. Keep the error on the listing state so a failed
          listing is distinguishable from an empty one; the next list
          clears it.