Keep a DiffView hunk's collapse with that hunk, not its position [patch]

[matt-edmondson]

Fixes #482

What was wrong

DiffViewState stored collapsed hunks as a HashSet<int> of positions. That state lives as long as the widget's label, so:

1. Collapse hunk 2 of file A, then select file B under the same label, and B's hunk 2 opens collapsed.
2. Stage hunk 0 and re-read the diff. Every later hunk shifts down one index, so the collapse marker lands on a different hunk.

Change

This follows the issue's preferred fix. The collapse flag is now a Collapsed property on HunkLayout. LayoutFor already replaces a layout when a different hunk instance arrives at a position (!ReferenceEquals(layout.Hunk, hunk)), so:

• a new hunk at an old position starts expanded;
• the same hunk instance redrawn keeps its state.

IsCollapsed and ToggleCollapsed now take the hunk as well as the index. Both are internal, and DiffView's three call sites already have the hunk in scope. The HashSet<int> is gone.

The issue's optional suggestion was left out on purpose: the static States dictionary is still never evicted. It grows only per distinct label, and nothing in this PR adds to it.

Tests

DiffViewStateTests:

• Collapse_ADifferentHunkAtTheSamePosition_StartsExpanded is the bug. Temporarily switching the state back to index-only storage makes it fail, and it passes with the fix.
• Collapse_TheSameHunkAgain_StaysCollapsed covers the issue's second requested case.
• Collapse_OtherPositions_AreUnaffected checks that collapsing one position leaves the others alone.
• Collapse_ToggledTwice_ReturnsToExpanded is updated for the new signature.

Results:

• ImGui.Widgets.Tests: 1126/1126 pass.
• dotnet build ImGui.Widgets across all target frameworks: 0 warnings, 0 errors.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JrNRgZ65vSm9BQxYHcanRS

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
87.5% Coverage on New Code
0.0% Duplication on New Code

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