feat(theme): make Boreal the distro default

Seed theme-state.json and the downstream template from themes/boreal.json
(including border + empty settings). Point eval/schema fallbacks at Boreal
(theme.nix preset, plymouth and live-ISO splash colors, theme-shot, sync
error examples). Docs/TESTING and RECOVERY follow the new default.

Verified: state matches boreal palette; theme-contrast all pass.
preview.png for boreal still missing (picker plain-name; V3 capture).

pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py

@@ -208,7 +208,7 @@ def validate_state(state) -> tuple:
 
     if not isinstance(state, dict):
         err("top level", "must be a JSON object",
-            "re-apply a preset: nomarchy-theme-sync apply tokyo-night")
+            "re-apply a preset: nomarchy-theme-sync apply boreal")
         return errors, warnings
 
     for k in state:
@@ -221,7 +221,7 @@ def validate_state(state) -> tuple:
                         ("icons", "an icon theme name (or \"\" for pick-by-mode)")):
         v = state.get(field)
         if v is not None and not isinstance(v, str):
-            err(field, f"must be a string ({want}), got {v!r}", 'quote it, e.g. "tokyo-night"')
+            err(field, f"must be a string ({want}), got {v!r}", 'quote it, e.g. "boreal"')
 
     mode = state.get("mode")
     if mode is not None and mode not in ("dark", "light"):