docs: correct package names and audit documentation

[matt-edmondson]

Summary

The documentation referenced the legacy, dot-less NuGet package IDs (ktsu.ImGuiApp, ktsu.ImGuiWidgets, ktsu.ImGuiStyler, ktsu.ImGuiPopups), which are abandoned at v1.x/2.1.x. The packages the repo actually builds and publishes today (v2.5.0) use dotted IDs: ktsu.ImGui.App, ktsu.ImGui.Widgets, ktsu.ImGui.Styler, ktsu.ImGui.Popups (confirmed via the ktsu.Sdk-derived PackageId and the namespaces in source). This PR updates every install snippet, badge, and namespace in the docs to the correct names, and audits the rest of the documentation for accuracy while doing so.

The per-library READMEs turned out to be stale copies from the original standalone repos, so the audit caught a number of code examples referencing APIs that no longer exist (or never did).

Package name / namespace fixes

• All dotnet add / Install-Package / <PackageReference> snippets and NuGet badges across README.md, CLAUDE.md, and the four per-library READMEs.
• using directives in code examples updated to the shipped namespaces (ktsu.ImGui.App, etc.).
• The repo/suite brand name in the root README H1/intro (matching ktsu-dev/ImGuiApp) was intentionally left as-is.

Audit fixes (verified against source)

Styler

• Theme.Reset() → Theme.ResetToDefault()
• Removed non-existent Theme.IsCurrentThemeDark
• Color.FromHSV → Color.FromHSL
• Color.Lighten/Darken/GetOptimalTextColor were never static Color methods — documented as the actual ImColor extension methods (LightenBy, DarkenBy, CalculateOptimalContrastingColor, GetContrastRatioOver)
• new ScopedTheme("string") → new ScopedTheme(ISemanticTheme)
• Standardized theme names to the space-separated form the registry/demo use

Widgets

• Icon example: IconDelegates → IconOptions, dropped the non-existent Color parameter, use ImGuiAppTextureInfo.TextureId
• Grid example: RowMajorGrid with an id and the correct CalcIconSize overload
• ColorIndicator argument order/types corrected
• DividerContainer: Columns/Rows, Add(), Tick() (was Horizontal/Vertical, AddZone(), Draw())

Popups

• FilesystemBrowser.Open(mode, target, …) → FileOpen / FileSave / ChooseDirectory
• Corrected dependency list (ktsu.Semantics.Paths/Strings, not ktsu.StrongPaths)

App / Root

• Supported frameworks corrected to .NET 8/9/10
• EmsToPx/PtsToPx documented as returning int
• Qualified nested widget types (ImGuiWidgets.TabPanel, etc.)
• Standalone clone/demo commands pointed at this repo

Testing

Docs-only change; all referenced APIs were verified against the current source.

https://claude.ai/code/session_013SjKypvXUFBVfTVrfCxt9D

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