fix(file-panels): Settings offers Source where the picker does, and review follow-ups

- Settings > Files offers Source on every key except those a built-in panel of
  another kind (Image) claims, so a plugin that alone claims a text extension
  shows the same choices as the picker.
- Delete the unused isSourceFilePanel.
- The Markdown example warns with the file name when its unmount write fails,
  rather than dropping the edit silently.
- Document that PluginFile stat fields are read at render time and that watch
  or the read hooks are how a panel learns they changed.

public-docs/plugins/reference.md

@@ -1806,7 +1806,9 @@ props described under [Theme and layout](#theme-and-layout), plus:
 | `location`    | `PluginFileLocation` | `lineStart?`, `lineEnd?`, and `revision`, which increments on every navigation. |
 
 Paseo mounts your panel once it knows the file exists and its size, without reading it. The
-`PluginFile` stat fields refresh when the file changes on disk:
+`file` object is stable, and its stat fields hold the latest version Paseo knows. Read them while
+rendering; changing them does not re-render your panel. A `watch` listener, or a re-render from
+`useFileText`, `useFileBytes`, or `useFileUrl`, is how your panel learns they changed:
 
 | Member                           | Meaning                                                                                     |
 | -------------------------------- | ------------------------------------------------------------------------------------------- |