Highlight selected feature inputs and dependents

[altendky]

Selecting one or more features now adds subtle row highlights for their upstream inputs and downstream dependents, including indirect relationships. Cool and warm tints distinguish the directions; selected rows keep their stronger highlight. Collapsed branches indicate related features inside, with tooltips and accessible descriptions explaining the relationship.

Follow the same declared references and generated ownership used by reorder validation. Keep drop validation unchanged. Show a compact color key without moving rows when selection changes, preserving double-click editing.

Validation:

• All 268 browser unit checks passed, including closure direction, multiselection, exact output contexts, generated ownership, cycles and selected-row precedence.
• All 15 Chromium feature-tree tests passed, including selection/clearing, collapsed branches, stable row positions, double-click editing and multiselection dragging.
• Browser build and pre-commit checks passed.
• Visually inspected the highlights against the running repeated-boss example.