Say the folder dialogue is the portal, and what the Flatpak has not proved

distribution.md §4, outstanding.md's FR-PLAT-LIN-3 entry, the Flatpak
manifest's comment and the README all said a library was chosen by
typing a path and that nothing in the tree called the FileChooser
portal. Since 6683c14 every folder the desktop asks for is chosen
through rfd's xdg-portal backend, so those sentences were false.

What they now say instead is narrower than "it works in the sandbox":
no Flatpak has been built here, so whether the portal's path opens a
library, holds across a restart and takes a sidecar is unobserved, and
volumes() still cannot see a host card. The chooser also landed in ui/
rather than behind the dr-plat seam distribution.md had proposed, and
both documents say so. FR-PLAT-LIN-3 gets a status note to the same
effect.

packaging/flatpak/paris.tourolle.darkroom.yml

@@ -90,17 +90,21 @@ finish-args:
   # path in argv and opens. That path is genuinely portal-mediated and needs no
   # code change.
   #
-  # What that leaves broken: choosing a *library root*. The folder connector
-  # takes a typed absolute path (`SignIn::EndpointOnly`, placeholder
-  # `/home/you/Pictures`) and checks it with `std::fs`, and nothing in the tree
-  # calls the FileChooser portal — there is no ashpd, no rfd, no toolkit dialog.
-  # A path typed into that field does not exist in this sandbox, so the launch
-  # screen refuses it with "that folder does not exist", which is at least an
-  # honest error.
+  # What that leaves to the portal: choosing a *library root*, an import's
+  # source and every other folder the desktop asks for. Since 0.17.0 they are
+  # chosen in `ui/dr-ui/src/folder_dialog.rs`, which asks the FileChooser
+  # portal through rfd (no talk hole needed: portals are always reachable),
+  # and a folder chosen there arrives as a document-portal path under
+  # /run/user/$UID/doc. That is the design; no Flatpak has been built from
+  # this manifest yet, so it has not been seen to work in the sandbox.
+  # Removable cards are still invisible to `dr_plat::volumes()`, which reads
+  # the sandbox's own mount table; the import page's Browse… reaches one
+  # through the same portal.
   #
-  # `--filesystem=host` would make that work today and is exactly what the
-  # requirement forbids, so it is not here. docs/dev/distribution.md §4 records what
-  # closes the gap and how to run a Flatpak build in the meantime.
+  # `--filesystem=host` would make all of it work without the portal and is
+  # exactly what the requirement forbids, so it is not here.
+  # docs/dev/distribution.md §4 records what is left to prove and how to run a
+  # Flatpak build in the meantime.
 
 modules:
   - name: darkroom