GTK4: 4 styling FFI gaps surfaced by #185 Phase B audit

[proggeramlug]

Context

The styling matrix added in #185 Phase A (crates/perry-ui/src/styling_matrix.rs) audits which styling FFIs are wired on each backend. After Phase B closures (v0.5.297–v0.5.303), every Apple platform + Android + Web is at 43/43 Wired. GTK4 sits at 39/43, with 4 real implementation gaps that can't be reasonably verified from the macOS dev host (no GTK4 toolchain) and warrant a focused Linux-side closure.

The 4 Missing rows

Generated from docs/src/ui/styling-matrix.md:

Prop	FFI symbol	Recommendation
widget.on_click	perry_ui_widget_set_on_click	widget.add_controller(GestureClick::new()) with connect_pressed invoking the registered closure
button.content_tint_color	perry_ui_button_set_content_tint_color	CSS color override on Button > image selector via per-handle CssProvider (mirrors set_corner_radius pattern)
button.image_position	perry_ui_button_set_image_position	Button::set_child(&Box) with image+label ordered by direction; or Button::child().downcast::<Box>().reorder_child_after()
stack.detaches_hidden	perry_ui_stack_set_detaches_hidden	GTK4 already auto-detaches hidden children from layout; could be a no-op stub OR reuse Box::set_visible cascade — needs Linux verification

What's needed

For each row above:

1. Implementation in crates/perry-ui-gtk4/src/widgets/mod.rs (or appropriate sub-module — button.rs for the button-specific ones).
2. FFI export in crates/perry-ui-gtk4/src/lib.rs — #[no_mangle] pub extern \"C\" fn perry_ui_<symbol>(...).
3. Matrix flip in crates/perry-ui/src/styling_matrix.rs from Missing → Wired for the GTK4 cell. Drift integration test (cargo test -p perry-ui) catches mismatches at CI time.

Why a separate issue

The macOS dev host can't easily cross-compile GTK4 (pkg-config for pango/gtk via system, libmimalloc-sys build issues). The CI ubuntu-22.04 runner has GTK4 system libs and can build cleanly. A Linux contributor or a CI-only PR is the natural path.

Verification

Once landed, cargo test -p perry-ui must remain green (the styling matrix integration test scans crates/perry-ui-gtk4/src/lib.rs for the new FFI exports and fails on drift).

Quick verification flow:

# After implementing each FFI export:
./target/debug/styling-matrix --check    # validates lib.rs vs matrix
./target/debug/styling-matrix --gen      # regenerates docs/src/ui/styling-matrix.md
git diff --exit-code -- docs/src/ui/styling-matrix.md  # CI guard

Closes

Once landed: GTK4 reaches 43/43 Wired alongside Apple+Android+Web. Only Windows-deferred-paint family (5 stubs) remains for full Phase B styling closure on issue #185.