Implement the keyboard grammar and full command map

The board's fixed grammar keys and the menu-backed chords of
04-interactions.md § Keyboard, per the Command Nexus inventory:

- Spatial arrow navigation (NavigationMath.nearest over the marquee
  registry's frames — one geometry source), walking across interior
  masonry columns, lanes, and into the shown trash; ⇧-arrows extend via
  the same range function as ⇧-click and go inert at the liveness and
  kind boundaries; ⌥-jumps with the ⌥↑ lane-domain escalation and ↓
  descent; the empty selection seeds at the first lane's first card;
  selection scrolls into view.
- selectionHead — the navigation cursor beside the anchor, set by every
  click, moved by every arrow, dropped by the reload vanish rule.
- Board ▸ Open Card ⌘↩ (the one command enabled mid-edit: commits the
  placeholder or rename and opens), Move Up/Move Down ⌥⌘↑/⌥⌘↓
  (within-lane sort, gather-then-step, rank-permuting writes in one
  bracket), Move Left/Move Right ⌘←/⌘→ (sole lane, one slot, never the
  trash) — all validating and acting off one shared answer.
- Delete now selects the Finder-style successor sibling from the
  pre-write snapshot, so repeated ⌫ walks down a lane; external
  vanishing still only shrinks the selection.
- handleReturn rejects modified Returns; the trash column renders
  eagerly so every row stays registered for navigation and the marquee.

686 unit tests (27 new).

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/UI/Board/SelectionClicks.swift

@@ -68,9 +68,13 @@ struct MarqueeControl {
                 if ids.isEmpty {
                     store.clearSelection()
                 } else {
-                    // No anchor: a band names no click to range from, so a ⇧-click after one acts
-                    // plain (`TransientBoardState.selectionAnchor`).
-                    store.select(ids, liveness: session.side, anchor: nil)
+                    // Neither cursor: a band names no click to range from and no item to arrow from,
+                    // so a ⇧-click after one acts plain and an arrow re-derives a position from the
+                    // set's last member (`TransientBoardState.selectionAnchor`, `selectionHead`).
+                    // Both are spelled out rather than defaulted, because `select`'s sole-member
+                    // default would otherwise pick one up the moment a band happened to sweep
+                    // exactly one card.
+                    store.select(ids, liveness: session.side, anchor: nil, head: nil)
                 }
             }
             .onEnded { _ in session.end() }