feat(home): mkDefault behaviour settings so downstream can plain-override

Nomarchy's Hyprland/Ghostty/Waybar settings were set at normal priority,
so a downstream `home.nix` setting any value Nomarchy already set threw a
"conflicting definition values" error — overriding required lib.mkForce,
contradicting the README's "mkDefault throughout" claim.

Now: behaviour/chrome leaves (input, misc, monitor, animations, dwindle,
gesture, decoration.blur sizing, ghostty padding/decoration, waybar
settings+style) are lib.mkDefault — a plain home.nix assignment wins.
Appearance values that flow from theme-state.json (gaps, colors,
rounding, opacity, fonts) stay at normal priority on purpose: change them
via nomarchy-theme-sync (their source of truth) or lib.mkForce to
hardcode. bind/exec-once lists stay normal priority so additions
concatenate rather than replace.

Verified: behaviour/ghostty plain overrides build; theme values still
conflict (steering to the CLI); default rendered config is byte-identical.

New docs/OVERRIDES.md with the appearance→CLI / behaviour→home.nix /
component→toggle model and worked examples; linked from the README.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/waybar.nix

@@ -145,14 +145,21 @@ in
     enable = true;
     systemd.enable = true; # started/stopped with graphical-session.target
 
-    settings.mainBar =
+    # mkDefault so downstream can replace the whole bar config/style with
+    # a plain home.nix assignment. For per-theme identity, prefer the
+    # whole-swap assets (themes/<slug>/waybar.{jsonc,css}); for one-off
+    # tweaks of the generated bar, lib.mkForce a sub-key. See
+    # docs/OVERRIDES.md.
+    settings.mainBar = lib.mkDefault (
       if hasConfigOverride
       then builtins.fromJSON (builtins.readFile configOverride)
-      else generatedSettings;
+      else generatedSettings
+    );
 
-    style =
+    style = lib.mkDefault (
       if hasStyleOverride
       then builtins.readFile styleOverride
-      else generatedStyle;
+      else generatedStyle
+    );
   };
 }