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>

README.md

@@ -66,6 +66,7 @@ Flat on purpose. Two module trees, one options file each, no hidden layers.
 │   └── nomarchy-install/          # live-ISO installer (gum + disko + mkFlake)
 ├── templates/downstream/          # `nix flake init -t` starter for users
 ├── docs/TESTING.md                # how to verify changes (incl. AI-agent rules)
+├── docs/OVERRIDES.md              # how downstream users override defaults
 └── tools/                         # maintainer-only
     ├── import-palettes.py         #   converts old-distro themes → JSON + assets
     ├── test-live-iso.sh           #   build the ISO + boot it in QEMU
@@ -154,8 +155,11 @@ sys-update    # nix flake update + system rebuild (BTRFS snapshot first when ava
 home-update   # home-manager switch (no flake update, no sudo)
 ```
 
-Override anything with plain NixOS/HM options (the distro uses `mkDefault`
-throughout) or the `nomarchy.*` surface:
+Override anything via the `nomarchy.*` surface or plain NixOS/HM options:
+appearance (gaps/colors/fonts) changes through `nomarchy-theme-sync`,
+behaviour (input/misc/monitor/chrome) is `mkDefault` so a plain `home.nix`
+assignment wins, and bind/exec-once lists concatenate. Full guide with
+examples: **[docs/OVERRIDES.md](docs/OVERRIDES.md)**.
 
 | Option | Default | Purpose |
 |---|---|---|