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/NewCardTarget.swift

@@ -20,11 +20,11 @@
 ///
 /// ### What it deliberately does not decide
 ///
-/// - **The lane header's new-card button overrides this rule entirely** (11-command-nexus.md ▸
-///   Pointer grammar, settled): "the click names its target lane, selection notwithstanding". That
-///   call site passes its own lane and never comes here.
 /// - **Return on a selected lane** is the same target as this rule's lane branch, but it is reached
-///   by grammar rather than by the menu; it also passes its lane directly.
+///   by grammar rather than by the menu; it passes its lane directly rather than coming here. (The
+///   lane header once carried a new-card button with the same carve-out — click names its target,
+///   selection notwithstanding — but the button itself is gone; Return is the one direct-target path
+///   left.)
 enum NewCardTarget {
 
     /// Where a new card goes: which lane, and which card it lands immediately after (`nil` = the