fix(windows): restore rounded leading navigation surface

[LimiNode]

Summary

• restore the NavigationStrip leading item's rounded bottom-left presentation;
• make the title-region own the stable bottom-left corner while the side seam stays square;
• keep activation, keyboard navigation, and text layout delegated to Dear ImGui's MenuItem;
• use Dear ImGui's actual item rectangle for the optional leading-item mask;
• keep title-bar geometry and theme colors owned by ImGuiX.

Verification

• cmake --build external/ImGuiX/build-mingw --target imguix --parallel 36
• cmake --build build-mingw --target mgc-gui --parallel 36
• ImGuiX CTest: 15/15 passed
• mgc-platform CTest: 17/17 passed
• manual PrintWindow smoke capture confirms the rounded System surface

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: d3967dffab

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Keep the navigation-strip seam square

When the leading navigation item is selected, hovered, or held, this manually painted bottom-left-rounded surface replaces the regular MenuItem header and makes the title/body junction rounded. The canonical NavigationStrip behavior requires that junction to remain square and explicitly relies on the regular themed MenuItem surface rather than a second rounded overlay, so this produces incorrect chrome in every active first-item state; remove the overlay and retain the normal header rendering.

AGENTS.md reference: AGENTS.md:L265-L273

Useful? React with 👍 / 👎.