#2889 gatorwalk-factory studio: a factory whose model file is briefly invalid YAML drops off the list, and the page jumps to another factory

Found while checking #2808 (Simulate mode) in a browser.

While a factory's model definition file does not parse as YAML (an agent mid-edit, or a bad indent), swamp's model listing leaves that factory out, so the studio's /api/factories omits it. On the next relist the page receives a factories event, cannot find the selected factory, and selects the first remaining one (state.ts reloadFactories / loadFactories).

Effect:

• Design mode never shows the YAML problem for that file. It has a STALE path for a file that fails the schema, but this file vanishes instead.
• Simulate mode loses an open walk without the confirm it asks for on every other switch.
• When the file is fixed, the factory comes back, but the page stays on the other one.

Repro: serve the studio on a repo with two factories, open factory A, break A's indentation so the file is not valid YAML, and wait 3 seconds. The picker is now on B. curl /api/factories lists only B until A is fixed.

Expected: the page keeps the selected factory while its file is broken and shows the YAML error, as it does for a schema error. One way is for listFactories to keep a factory whose definition file swamp cannot parse, reported with an error, or for the page to keep its selection when the selected name merely disappears.