fix(navigationview): reconcile menu items in place to preserve expansion

[codemonkeychris]

Problem

NavigationView with hierarchical menu items mis-behaved on every interaction:

• clicking a category expanded it then immediately collapsed it (a flash);
• clicking a child item collapsed its parent;
• selection state churned on each render.

Root cause

UpdateNavigationView (and the V1 NavigationViewDescriptor) did
MenuItems.Clear() + full rebuild whenever the MenuItems array reference
changed. Consumers typically build a fresh MenuItems array each Render()
(e.g. via LINQ), so the ReferenceEquals guard never held and the rebuild
fired on essentially every re-render.

Per-item expansion lives only on the live NavigationViewItem — there is no
IsExpanded on NavigationViewItemData — so recreating the containers reset
expansion to false every time. Because selecting an item triggers a
re-render, the just-expanded category (or a child's parent) snapped shut on the
same click.

Fix

Reconcile menu items in place instead of clear-and-rebuild:

• match existing NavigationViewItems by Tag;
• update only changed Content / Icon;
• recurse into children;
• reuse the container so its runtime IsExpanded (and selection) survive the
  re-render.

A fast path skips all detaching when the item structure is unchanged (the
common case). Mirrored into the V1 NavigationViewDescriptor so V1 ON behaves
identically to V1 OFF. Mount-time selection now recurses so a child tag selects
correctly.

Tests

• New E2E guard: NavigationView_Hierarchical host fixture +
  NavigationViewInteractionTests. Verified to fail on the pre-fix build
  (expanded category collapses after the selection re-render) and pass after.
• Existing NavigationView self-tests, including the V1
  Desc_NavigationView_MountUpdate coverage, pass (0 failures).
• Also adds a TreeView E2E guard exercising the Gallery's basic text-tree path.

Out of scope

Compact (icon-only) pane interaction is deferred to a follow-up.

[Copilot]

Pull request overview

This PR fixes hierarchical NavigationView updates so menu item containers are reconciled in place instead of being cleared and recreated, preserving runtime expansion and selection state across re-renders.

Changes:

• Adds in-place NavigationView menu reconciliation in both legacy and V1 descriptor paths.
• Updates mount-time selection to find nested items by tag.
• Adds E2E fixtures/tests for hierarchical NavigationView and TreeView interaction regression coverage.

Show a summary per file

File	Description
src/Reactor/Core/Reconciler.Update.cs	Replaces clear-and-rebuild menu updates with Tag-based in-place reconciliation.
src/Reactor/Core/Reconciler.Mount.cs	Recursively resolves initial SelectedTag for nested NavigationView items.
src/Reactor/Core/V1Protocol/Descriptor/Descriptors/NavigationViewDescriptor.cs	Mirrors in-place menu reconciliation for V1 NavigationView behavior.
tests/Reactor.AppTests/Tests/NavigationViewInteractionTests.cs	Adds E2E coverage for preserving hierarchical NavigationView expansion after selection re-renders.
tests/Reactor.AppTests/Tests/TreeViewInteractionTests.cs	Adds TreeView interaction regression tests.
tests/Reactor.AppTests.Host/Fixtures/NavigationViewE2EFixtures.cs	Adds the hierarchical NavigationView host fixture.
tests/Reactor.AppTests.Host/Fixtures/TreeViewE2EFixtures.cs	Adds the basic text TreeView host fixture.
tests/Reactor.AppTests.Host/FixtureRegistry.cs	Registers the new E2E fixtures.

Copilot's findings

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• Files reviewed: 8/8 changed files
• Comments generated: 2

[codemonkeychris]

Addressed the review feedback in dd7ce54:

• Duplicate-key reuse (correctness): the slow-path rebuild now consumes the reuse-map entry via Dictionary.Remove(key, out …) when a container is reused. Duplicate sibling keys — duplicate Tags, or duplicate Content when no Tag is set — now fall through to a freshly created container instead of adding the same WinUI item to the collection twice. Applied to both the legacy arm and the V1 descriptor.
• Explicit filtering: the snapshot loops now filter explicitly with .OfType<…>() / .Where(…).

Verified: NavigationView self-tests (incl. Desc_NavigationView_MountUpdate) pass with 0 failures, and the NavigationViewInteractionTests E2E guard still passes (and still fails on the pre-fix build).