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

KanbanTests/AccessibilityPhrasesTests.swift

@@ -6,7 +6,7 @@ import Testing
 /// the tree as sentences:
 ///
 /// > A lane container is labeled "⟨title⟩, lane, N cards" — the count reads the search filter like
-/// > the visible badge. The lane header's new-card button is a labeled child ("New card in ⟨lane⟩").
+/// > the visible badge. The lane header's collapse chevron is a labeled child ("Collapse ⟨lane⟩").
 /// > A card is one flattened element: label = title (or the untitled placeholder), value carries the
 /// > attachment count when present, selected state via trait … cut cards expose their dimmed pending
 /// > state in the value ("cut, pending paste").
@@ -69,18 +69,6 @@ struct AccessibilityPhrasesTests {
         }
     }
 
-    // MARK: - The new-card button
-
-    @Test("The header button names its lane")
-    func newCardLabel() {
-        #expect(AccessibilityPhrases.newCardLabel(lane: "Doing") == "New card in Doing")
-    }
-
-    @Test("The header button of an untitled lane names the placeholder")
-    func newCardLabelUntitled() {
-        #expect(AccessibilityPhrases.newCardLabel(lane: nil) == "New card in Untitled")
-    }
-
     // MARK: - Card elements
 
     @Test("A card's label is its title")