* fix(compiler): defer open until successful compile, close viewer on :bd
Problem: For long-running providers (e.g. `typst watch`), the viewer
was opened immediately on toggle start by checking if the output file
existed on disk. A stale PDF from a prior session satisfied that check,
so a failed compile still opened the viewer. Additionally, viewer
processes spawned via a table `open` command were untracked, so `:bd`
killed the compiler but left the viewer running.
Solution: Replace the immediate open with a `vim.uv.new_fs_event`
directory watcher that fires only when the output file's `mtime`
advances past its pre-compile value, proving the current session wrote
it. Add `viewer_procs` and `open_watchers` tables with `close_viewer`
and `stop_open_watcher` helpers; all `BufUnload` paths and `stop_all`
now tear down both. Extract `do_open` to deduplicate the open branching
logic across three call sites.
* docs: document viewer auto-close behaviour and limitations in `open` field
* ci: format
239f8a4769