fix: Explorer-style tile-pane arrows, Page, and Tab (0.8.4-beta1)

[piranout]

What

Keyboard navigation in the tile pane did not match Explorer.

Up/Down used the existing MoveGrid path, which walks a linear NavStop list (headers + tiles) by ±columns. In a wrapping, date-sectioned grid that looks random: short last rows throw the index off, and Down often lands on the next section header instead of the tile in the same column.

PageUp/PageDown were not handled at all, so ListBox default (skip N items / jump to a header) ran.

Tab walked the ribbon (hamburger, command bar, search) and OnEverythingKeyDown selected the first tile when entering the grid. WPF ListBox/TreeView also select the first item on focus.

Fix

• Extracted TileSpatialNavigation: layout-based 2D pick over wrap + section gaps (headers are not cells).
  • Up/Down: same visual column on the adjacent row. Empty cell (short last row, end of a section): nearest in that column, then nearest in the row. Crossing a date-section header is fine.
  • PageUp/PageDown: scroll one viewport, then the tile in the same column closest to the same viewport Y. Not skip-N-items. After layout, a realized-container refine corrects header-height drift.
• Left/Right still walk visual order (including headers). Home/End unchanged.
• Tab / Shift+Tab: KeyboardNavigation.TabNavigation is Once on the folder pane and the tile ListBox, None on the ribbon / status / preview. Search stays on / and Ctrl+F.
• Tabbing into a pane does not change its selection (LibraryPaneFocus.ShouldKeepExistingSelection + PreviewGotKeyboardFocus). Canvas-click focus (LibraryPaneFocus / ItemsList.Focus() from PR fix: focus the tile pane when clicking its empty canvas (0.8.3-beta1) #4) is unchanged; spatial arrows and Page keys work after that focus.

Version is 0.8.4-beta1 in Aperture.App.csproj. CHANGELOG has ## [0.8.4-beta1].

How to test

1. dotnet test tests/Aperture.Core.Tests/Aperture.Core.Tests.csproj — TileSpatialNavigationTests (same column, page = one viewport, not skip-N) and LibraryPaneFocusTests (canvas claim + Tab must not move selection). Live WPF is not unit-tested.

2. Manual path (the real check):

   1. Tree arrows keep the selected folder.
   2. Tab to the grid — the selected tile (or none) stays; it must not jump to the first tile.
   3. Up/Down stay in the same visual column, including across a date-section header. A short last row does not jump to a random index or a header.
   4. PageUp/PageDown move ~one screen and keep the same column at ~the same screen Y, at any zoom.
   5. Tab back to the tree — the same folder stays selected.
   6. Ribbon (hamburger / command bar / search) is never a Tab stop. Search is still / and Ctrl+F.
   7. Click the empty tile-pane canvas (PR fix: focus the tile pane when clicking its empty canvas (0.8.3-beta1) #4) — arrows and Page keys still apply to the grid.

Notes

• No CI/signing rewrite, visibility unchanged.
• After merge, auto-tag should create v0.8.4-beta1.