refactor(ui): extract MainForm startup and settings services

Moves startup and settings code out of MainForm:
- PostProcessorLoader discovers posts and registers their materials.
  Each DLL now loads in isolation; a failing one is named in a startup
  warning instead of throwing out of the MainForm constructor.
- MachineSettings owns the clamp-settings and nest-defaults entry points
  and their once-per-session warnings; BomImportForm calls it directly.
- NewNestFactory creates File > New nests and their daily names;
  GetNestName is now a pure static the characterization test calls.
- MainWindowSettings restores/saves window placement and migrates the
  legacy nest template.
- UpdaterStatus owns the updater and its status-bar item.

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

OpenNest.Data/NestDefaults.cs

@@ -49,7 +49,7 @@ public sealed class NestDefaults
     /// <summary>
     /// The built-in defaults used when no file exists and for every field
     /// that is missing or invalid. Matches the historical
-    /// MainForm.CreateDefaultNest values (units default to Inches; callers
+    /// File > New values (units default to Inches; callers
     /// may override from their own settings).
     /// </summary>
     public static NestDefaults Fallback => new();