Add InkCanvas and InkToolBar control pages

[Nitin-100]

Summary

Adds Gallery demo pages for the new InkCanvas and InkToolBar lifted controls being introduced in WinAppSDK 2.0 / WinUI 3. These pages let users explore inking end-to-end inside WinUI Gallery alongside the existing controls.

Pairs with the corresponding microsoft-ui-xaml-lift PR that ships the controls themselves: PR #15101676 (user/nitinc/inking-support → main).

What's added

File	Purpose
WinUIGallery/ControlPages/InkCanvasPage.xaml (+378)	InkCanvas demo UI — pen / pencil / highlighter strokes, sample InkPresenter wiring
WinUIGallery/ControlPages/InkCanvasPage.xaml.cs (+774)	Code-behind covering basic inking, custom drying, save/load, eraser, custom pen samples
WinUIGallery/ControlPages/InkToolBarPage.xaml (+76)	InkToolBar demo UI — built-in buttons, stencils, custom buttons
WinUIGallery/ControlPages/InkToolBarPage.xaml.cs (+35)	InkToolBar interactions and event wiring
WinUIGallery/DataModel/ControlInfoData.json (+19)	Registers the new "Inking" section in left-nav
WinUIGallery/WinUIGallery.csproj (+2)	Includes the new pages
Directory.Build.props, nuget.config (+3)	Minor build/package config tweaks needed for the inking sample

Total: 8 files, +1,287 lines, all additive — no existing Gallery files modified destructively.

Validation

Built and ran locally on Windows 11 (amd64chk WinUI Gallery layout) against the matching lifted-controls DLL from PR #15101676. End-to-end checks pass:

• Pen / pencil / highlighter draw strokes on the canvas
• InkToolBar pen / eraser / color / size controls work as expected
• Custom drying pipeline runs (BeginDry / EndDry cycles fire)
• Independent input thread receives PointerPressing / Moving / Releasing
• No threading errors (RPC_E_WRONG_THREAD) under switcher-engaged composition

Notes for reviewers

• Pages will only have functional inking when the matching WinAppSDK build with the new InkCanvas / InkToolBar controls is in use; against current shipping WinAppSDK they'll fail to instantiate the controls (expected until the lift PR ships).
• No changes to existing pages; navigation still works for everything else with the new pages absent or present.
• Happy to split this into per-control PRs (one for InkCanvas, one for InkToolBar) if reviewers prefer smaller chunks.

[Nitin-100]

Pairs with microsoft-ui-xaml-lift PR #15101676 which adds the InkCanvas and InkToolBar lifted controls these pages exercise.

[Nitin-100]

Holding this PR pending rebase onto upstream main after the WASDK 2.0 layout migration. Will resubmit on the new .slnx layout post-May-26 CC. Demo pages validated locally May 14 against the WinUI3 InkCanvas/InkToolBar implementation.