#2841 gatorwalk-factory studio: file-watch and page edge cases left from #2806's reviews

Follow-up to #2806 (the studio server). The verification reviews of #2806 raised these, and none of them blocked. The page is read-only, and reloading it recovers from each one.

Medium

• A replaced definition directory silences its watch. If an agent deletes and recreates a definition's directory (for example rm -rf factories && mkdir factories) between two relists, the watch set's key is unchanged, so refresh() does not watch it again. Live reload for that factory stops until the page reloads. Fix: when an event's path is itself a watched directory, or a watch's loop ends, clear the key and refollow (extensions/models/_lib/engine/studio_watch.ts).

Low

• The page does not resync after its event stream reconnects. An edit made while EventSource was reconnecting is missed. On open after an error, reload the list, the definition and the selected scenario (studio/src/app.ts).
• A symlinked scenarios/<factory> inside the repo is served but not watched. The watch skips any directory whose real path it has not confirmed. Watching the resolved directory would cover the in-repo case.
• A definition that becomes readable on a relist sends no definition event. An example is one whose refusal is lifted by a change outside the watched directories. The page keeps showing the error until the next event or a reload.
• The 3-second relist re-resolves every definition path even when the (name, path) list has not changed. Compare the list first and skip the file-system work when it is the same (studio_serve.ts and studio_watch.ts).
• Heavy refollow on churn. Every event on a directory that leads to an awaited definition rebuilds every watch. It is correct, but could rebuild only what changed.