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>

.gitignore

@@ -6,9 +6,9 @@ Publish/          # publish-winapp output on case-sensitive systems
 *.user
 .vs/
 
-# Local development assets
+# AI 協作素材不入 git
 For_AI/
 
-# CAD samples are stored locally in For_AI/
+# 客戶 CAD 料號檔不入 git（避免外流）；測試模型放 For_AI/
 *.stp
 *.step