phase-3: z-order — chrome overlays above WebKit subsurface

[mxaddict]

Wayland subsurfaces are stacked above their parent by default. WebKit's subsurface will cover any buffr-chrome pixels in the browser region. For things that need to overlay the page (omnibar dropdown, hint mode labels, find-in-page highlight bar, loading anim, permission prompt) we need them composited ABOVE the WebKit subsurface.

Approaches

A. Sibling subsurface for overlays

Create a second subsurface (`buffr_overlay_surface`) whose parent is the same winit wl_surface. Use `wl_subsurface::place_above(webkit_subsurface)` to stack it on top. Render overlays into this surface using a small wgpu offshoot OR a software-blit buffer.

B. Bring overlays to chrome strip only

Restrict overlays so they don't OVERLAY the browser region. Permission prompt + omnibar dropdown live in the chrome strip area where the chrome subsurface (= wgpu parent) is visible. Acceptable design constraint.

C. Don't composite WebKit when overlay active

Hide the WebKit subsurface (`wl_subsurface::place_below(parent)` is not a thing; use `set_visible`-style via a transparent buffer attach OR `wl_surface::commit` with a null buffer). Reveal when overlay closes.

Recommended

B for v1 — keeps Phase 3 scope manageable. File a follow-up for A once Phase 3 is stable.

Scope

• Audit all overlay code in apps/buffr-app/src/main.rs that paints into the browser region (`paint_chrome_strips` confirm strips only; `loading_anim::paint` — that's the big one).
• For `supports_native() == true`:
  • Skip loading-anim paint into the browser region (already covered by phase-3: alpha chrome — wgpu surface + chrome buffer make browser region transparent #146).
  • Constrain hint-mode labels to chrome strip (or accept they're invisible during Phase 3 v1 and file follow-up).
  • Permission prompt + omnibar dropdown already live in chrome — verify with grep, no code change expected.

Out of scope

• Sibling subsurface for above-page overlays (approach A) — separate follow-up ticket.

Acceptance

• Open omnibar over a Wayland-native WebKit page → dropdown appears.
• Hint mode → labels appear on chrome strip (if visible there) OR are deferred to follow-up.
• Loading anim doesn't fight with WebKit's subsurface.

Reference

• Parent: WPE WebKit backend (engine ID: webkit) — native Linux Wayland compositing #109 (Phase 3), depends on phase-3: create wl_subsurface from parent + attach WebKit's wl_surface as child #145 + phase-3: alpha chrome — wgpu surface + chrome buffer make browser region transparent #146