Host: 256 feature flags, the onboarding mode (mode_get/mode_set, shell-only), and the one-time move of replaced plugin files to feature flags

- settings: MAX_FEATURES 64 -> 256 (an older app's whole-map backup still validates)
- modes: ModeState {mode, onboarded, existing} in modes.json; existing is decided natively on the first start
- plugins::migrate: PluginHost.superseded (file -> flag); enabled and as shipped -> flag on and the file goes; edited -> kept, flag left off; marker written last, every step repeatable; map_backup for imported enabled lists
- a superseded file is never written again by install_bundled
- create-waybetter: the host command list and PluginHost.superseded

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/waybetter-host/src/commands.rs

@@ -28,6 +28,8 @@ pub const SHELL: &[&str] = &[
     "custom_css_set",
     "features_get",
     "features_set",
+    "mode_get",
+    "mode_set",
     "settings_get",
     "settings_set",
 ];