Validate DistroDatabase and split MainMenuController's starter/collection logic out

- DistroDatabase.TryValidate() catches empty slots, blank ids, and
  duplicate ids at edit time (OnValidate) and runtime, instead of only
  surfacing as a runtime count check.
- StarterSelectionController centralizes the "do we have a usable
  starter set" check (previously re-derived in five places) and shows a
  dismissible in-UI error when the database is misconfigured, rather
  than silently leaving the modal hidden with just a console warning.
- Boot-intro's bootIntroPlayed flag now resets via
  RuntimeInitializeOnLoadMethod so it stays session-scoped even when
  the Editor's Reload Domain setting is off.
- Extracted StarterSelectionController and CollectionScreenController
  out of MainMenuController (885 -> ~490 lines), following the same
  plain-class-plus-Bind() pattern already used by FeaturedUnitPanel.
  Shared DisplayName/FormatLanguages formatting and digit-key mapping
  moved into DistroPresentation/CommandKeyBindings to remove the
  duplication that split would otherwise have introduced.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01364Mm9NsPooiSfDHKr4FAx

Assets/Scripts/UI/DistroPresentation.cs

@@ -0,0 +1,20 @@
+using KernelPanic.Data;
+
+namespace KernelPanic.UI
+{
+    /// <summary>
+    /// Shared text formatting for distros, used by every screen that lists or details them.
+    /// </summary>
+    public static class DistroPresentation
+    {
+        public static string DisplayName(DistroDefinition unit)
+        {
+            return string.IsNullOrWhiteSpace(unit.DisplayName) ? unit.name : unit.DisplayName;
+        }
+
+        public static string FormatLanguages(DistroDefinition unit)
+        {
+            return $"{unit.PrimaryLanguage} / {unit.SecondaryLanguage}";
+        }
+    }
+}