PHLEGM v8 - Revision r97 - Single tree selection highlight

Behavior contract:
- Exactly zero or one tree item may have a selection outline at any instant.
- VIEW TREE + TYPEFACE CHAR: only current TreeView selection has a
  1-pixel dotted box following the CHR label (in MONO and ARGB).
- Other items are rendered with transparent CHR glyphs and no box.
- On selection change, the entire tree is invalidated asynchronously to
  clear the previous postpaint outline before drawing the new one.
- Painting snapshots TreeView_GetSelection ONCE per frame. A selection
  change during drawing cannot produce multiple targets in that frame.
- ICON follows the same single native selected item; FONT retains its
  native single-selection highlight.
- GTK explicitly sets GTK_SELECTION_SINGLE and updates only the previous
  and current label pixbuf outlines in place.
- This revision does not reintroduce black selection/backdrop fills.

Validation: targeted source/state assertions and package integrity.
Native FreeBASIC compilation and a live Windows/GTK selection-transition
visual regression test remain necessary.
