feat: merge nebula_gpui into the root workspace (crossfont 0.9.0 unblocks links)

- crossfont 0.8.1 -> 0.9.0: upstream bumped yeslogic-fontconfig-sys to ^6.0,
  aligning with gpui's zed-font-kit so a single 6.0.1 owns links=fontconfig.
- wayland CSD switches to the ab_glyph flavor so sctk-adwaita 0.10 stops
  pulling crossfont ^0.8 (and yeslogic ^5) back into the graph. Linux-only
  decoration text; Windows/macOS unaffected.
- Drop the workspace exclude and nebula_gpui's member-level lockfile; one
  lockfile now holds gpui 0.2.2 + crossfont 0.9.0. Both bins compile.
- ARCHITECTURE.md: the "unresolvable conflict" section now records the fix.

Cargo.toml

@@ -6,8 +6,8 @@ members = [
     "nebula_config_derive",
     "nebula-completions",
     "nebula_hook",
+    "nebula_gpui",
 ]
-exclude = ["nebula_gpui"]
 resolver = "2"
 
 [workspace.package]