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/BoardZoom.swift

@@ -199,7 +199,7 @@ extension View {
     /// change. Chrome outside the strip keeps using `.font(_:)` and stays at system size on purpose.
     ///
     /// SF Symbols need no equivalent — `Image(systemName:)` under `.imageScale(_:)` sizes off the
-    /// current font, which is why `BoardMetrics.newCardButtonReserve` is 1.7 em rather than 22pt.
+    /// current font, which is why `BoardMetrics.laneCollapseButtonReserve` is 1.4 em rather than 18pt.
     func boardFont(_ style: Font.TextStyle, weight: Font.Weight? = nil) -> some View {
         modifier(BoardFontModifier(style: style, weight: weight))
     }