Remove the lane header's "+" new-card control

The lane header no longer draws a new-card button. Every other
creation path is untouched: File > New Card (Cmd-N), Return on a
selected lane, and double-click on lane empty space.

- LaneView: drop the button and its overlay slot, leaving the
  collapse chevron as the header's one piece of trailing chrome.
- NewCardTarget: drop the button's click-names-target carve-out over
  the Cmd-N target rule (Return on a selected lane is now the only
  direct-target path left).
- AccessibilityPhrases: drop the button's spoken label.
- BoardMetrics: drop its width reserve; laneHeaderTrailingReserve now
  reduces to the collapse chevron's own reserve.
- DESIGN/03, 04, 10, 11: update the lane title-bar inventory, the
  Cmd-N override clause and search-clearing mechanism list, the
  accessibility tree-shape sentence, and the pointer-grammar row that
  described the button.
- Tests: drop the two pinned accessibility-label tests and the
  newCardButtonReserve assertions; retarget the truncation-headroom
  test at the now-solo laneHeaderTrailingReserve; fix stale doc-quote
  comments and the manual accessibility-verification checklist.

Full KanbanTests suite: 2807 tests, 482 suites, all passing.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/UI/Board/BoardView.swift

@@ -247,7 +247,7 @@ struct BoardView: View {
         // **The board is one tab stop** (10-accessibility.md ▸ Full Keyboard Access: "the board is
         // one tab stop with arrow-key navigation within"), and this is it: one focusable strip,
         // whose interior movement is the arrow grammar below rather than a focus stop per card.
-        // The controls *around* the cards — each lane's new-card button, the popovers, the toolbar —
+        // The controls *around* the cards — each lane's collapse chevron, the popovers, the toolbar —
         // are ordinary `Button`s and stay Tab-reachable in their own right, which is the other half
         // of the same sentence ("every control … is Tab-reachable").
         //