docs: restore Chinese design-rationale comments after i18n merge

PR #1 condensed the existing Chinese comments into shorter English,
dropping the 'why / don't revert to X' notes the project relies on
(merged-mesh face lookup, Identity-not-null transforms, background
section clip, ...). Restore the original comments verbatim where the
code is unchanged; new i18n code keeps its English comments. Two
FacesContent/MergedContent notes are corrected to the current render
modes. Comment-only change (verified: comment-stripped code identical).

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

tools/SmokeTest/Program.cs

@@ -1,4 +1,4 @@
-// Headless smoke tests for CAD import, triangulation, and the assembly tree.
+// 無 UI smoke test：驗證 CAD 檔解析 → 三角化 → 裝配樹 整條匯入管線
 using System.Diagnostics;
 using System.IO;
 using STPViewer.Services;