GTK4: perry_ui_open_folder_dialog not wired — blocks docs/examples/ui/dialogs/snippets.ts on Linux CI #216

Context

docs/examples/ui/dialogs/snippets.ts declares // platforms: macos, linux, windows and imports openFolderDialog from perry/ui to demonstrate the folder-picker pattern. The example compiles + links cleanly on macOS but fails on the GTK4-on-Ubuntu doc-tests CI runner with:

/usr/bin/ld: perry_llvm_*.ll:(.text+...): undefined reference to `perry_ui_open_folder_dialog'

Origin/main is currently red on the gtk4 doc-tests gate because of this and two layout FFIs (filed separately).

What's missing

Recommended GTK4 implementation

gtk::FileChooserDialog with Action::SelectFolder is the direct mapping. Mirror the existing GTK4 file-picker pattern (search crates/perry-ui-gtk4/src/ for FileChooserNative / FileChooserDialog) — fold this in by changing the action to SelectFolder and removing the file-filter setup. The TS callback signature is (path: string) => void; on cancel pass an empty string to match the macOS implementation's contract.

Acceptance

• perry_ui_open_folder_dialog exported as #[no_mangle] pub extern "C" fn in crates/perry-ui-gtk4/src/lib.rs.
• If a styling-matrix row applies to this FFI (it might not — folder dialog is more "system" than "styling"), flip the GTK4 cell from Missing to Wired.
• cargo build --release -p perry-ui-gtk4 succeeds.
• After re-running the doc-tests harness, docs/examples/ui/dialogs/snippets.ts compiles + links cleanly on Linux. Verify by inspecting the linker output (the example's runtime test mode exits before opening any modal, so PERRY_UI_TEST_MODE=1 is sufficient).
• No regression on the macOS doc-tests gate.

Pointers

• macOS implementation: search crates/perry-ui-macos/src/ for perry_ui_open_folder_dialog.
• GTK4 file-picker reference (closest existing analogue): search crates/perry-ui-gtk4/src/ for whatever already wires openFileDialog / saveFileDialog. Add the SelectFolder variant alongside.
• Failing doc-test: docs/examples/ui/dialogs/snippets.ts (banner declares all 3 platforms; line invokes openFolderDialog).

Related

• I hope it is easy and convenient to modify the component's style, similar to the style attribute in HTML. #185 (UI cross-platform audit)
• GTK4: 4 styling FFI gaps surfaced by #185 Phase B audit #202 (closed v0.5.317 — same shape, 4 GTK4 styling FFIs)

Estimated size

Small — ~30-50 LOC including the GTK4 FFI wrapper, the FFI export in lib.rs, and possibly an entry in the styling matrix.