Commit a34f015d78

v0.11: watch on OS filesystem events

Replaces a 500ms poll loop that re-walked the whole tree twice a second to compare
mtimes. Native events (notify) cost nothing while nothing happens and arrive in
milliseconds when something does. Write bursts are debounced over 120ms, because an
editor saving a file writes a temp file, renames it over the original and touches the
directory — one edit, several events.

What counts as a change is deliberately not the rule the build uses to find content:
- A build input is a change. Editing org-ssg.toml or a template rebuilds, even though
  discovery skips both as non-content. The question is "would this change the site?",
  not "is this a page?".
- Our own output is not. Dot-directories and editor scratch files are also out —
  including Emacs' file.org~ backups, which do not start with a dot and would otherwise
  look like content to a tool aimed at Emacs users.

Where native watching is unavailable (some network and container filesystems) it falls
back to polling and says so, rather than failing outright.

Two bugs, both found by mutation-testing the end-to-end test rather than by running it:

The first was in the test. `watch . -o _site` puts the output inside the source, so a
rebuild's writes raise events that trigger a rebuild forever, and the test asserted that
index.html's mtime held still. It does hold still during a runaway loop — the incremental
build leaves an unchanged page alone — so the assertion passed with the filter deleted.
It now watches syntax.css, which is rewritten on every build and is therefore a direct
record of how many builds have run.

With the assertion fixed, the unmutated code failed too, which is the second bug. On
macOS the temp directory is /var/…, a symlink to /private/var/…, and FSEvents reports the
resolved path. Stripping event paths with the root as the user typed it silently failed,
every event kept its absolute path, every absolute path looked like a source change, and
watch rebuilt in a loop. The filter now recognizes every spelling of the root, and a path
it cannot place is discarded rather than treated as a change.

Both mutations — deleting the output filter, and dropping the canonical root — are
verified to fail the test.

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