Replaces the plain python3 http.server + one-shot make blog with a
watcher pipeline: chokidar regenerates HTML on MD/template changes,
browser-sync serves the site and reloads the browser on rendered-asset
changes. First run downloads both via npx; subsequent runs are instant.
Preflight checks for npx and pandoc. Port arg parsing is tolerant of
legacy --drafts flag ordering (drafts are always included now, since
that's what the dev loop actually wants).
Cleanup trap kills the watcher on exit so re-runs don't leave orphans.
