Fix WFO1000 release build error on FormBase.HelpPage

The HelpPage property added for F1 online help (commit e127194) lacked
designer-serialization configuration. WFO1000 is error-severity by default,
and Crystallography.Controls intentionally does not suppress it (260322Ch),
so the managed release build (dotnet build -c Release) failed as soon as the
4.934 version bump triggered the release workflow. The error was pre-existing
and unrelated to the Media Foundation migration; it had simply never been
release-built because the workflow only runs on Version.cs changes.

Annotate HelpPage with [Browsable(false)] and
[DesignerSerializationVisibility(Hidden)], matching the CommonDialog.cs
pattern, since the property is set only in code (never via the designer).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: seto77

Crystallography.Controls/FormBase.cs

@@ -21,6 +21,10 @@ public partial class FormBase : Form
     /// このフォームに対応するマニュアルのページ識別子。HelpUrlResolver が URL を組み立てる際に使う。
     /// 値の意味 (スラッグ/番号など) はホストアプリの解決ロジックに依存する。
     /// </summary>
+    //260530Cl 追加: コードからのみ設定するプロパティなのでデザイナのシリアライズ対象外にする (WFO1000 回避)。
+    //         Crystallography.Controls は WFO1000 をプロジェクト単位で抑止しない方針(260322Ch)のため、CommonDialog.cs と同じく個別属性で対応する。
+    [System.ComponentModel.Browsable(false)]
+    [System.ComponentModel.DesignerSerializationVisibility(System.ComponentModel.DesignerSerializationVisibility.Hidden)]
     public string HelpPage { get; set; } = ""; //260529Cl 追加
 
     protected FormBase()