fix: panels stay above windows; don't force SSD on CSD-only (GTK) clients

[ebenali]

Fixes two related decoration/stacking bugs you hit with GTK windows and Waybar.

1. Windows occluded Waybar

Toplevels were direct children of the scene root, created after the per-output layer-shell trees, so every window rendered above the top/overlay layers where panels live. SSD windows showed it worst — their titlebar pokes above the client geometry into the panel's row — while CSD apps (Chromium, pcmanfm) have no overhang so they looked fine.

Fix: add a dedicated server->toplevel_tree and stack each output's layer trees around it — background/bottom below, top above, fullscreen above top, overlay on top. Toplevels are children of toplevel_tree, so focus-raising a window never lifts it above the panel layers. Fullscreen windows are reparented into the fullscreen layer (so they still cover panels) and back when restored.

2. GTK windows "mushroomed" on drag + double decorations

GTK never binds the xdg-decoration protocol — it is CSD-only, draws its own titlebar + shadow, and reports a shadow-offset xdg geometry (e.g. +14+12). We nonetheless imposed an SSD frame whenever DecorMode resolved to server-side, giving wbconf a double titlebar and broken drag geometry (the "mushroom").

Fix: gate SSD on the client actually participating in xdg-decoration. If a toplevel has no decoration object it's CSD-only and we leave it alone. Clients that bind the protocol (foot, most native Wayland apps) still get themed server-side frames and honour DecorMode.

Testing

• Full suite 27 green (g++/ASan/UBSan); clang + release clean.
• Headless: wbconf (GTK4) maps as CSD with no frame (no double decoration, no mushroom) while foot keeps its themed SSD frame; no crashes/ASan findings.
• Diagnosed live with temporary logging showing GTK4 deco=(nil) + shadow-offset geometry vs foot's real decoration object.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com