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/BoardMetrics.swift

@@ -115,35 +115,27 @@ enum BoardMetrics {
         em(0.4, bodyPointSize: bodyPointSize)
     }
 
-    /// The trailing room the header reserves for the new-card button, so **a long title truncates
-    /// before it collides with the button** rather than running under it.
+    /// The room the header reserves for the **collapse chevron**, its one piece of trailing chrome
+    /// (03-board-ui.md § Lane ▸ Collapsed lanes). A new-card button once shared this edge and had its
+    /// own matching reserve (`newCardButtonReserve`, retired with the button — this milestone); the
+    /// chevron kept its own figure rather than folding into a since-widowed combined one.
     ///
-    /// This is the one figure that is not decoration: the button is an `Image` at
-    /// `.imageScale(.small)`, which *is* a relative size, so a reserve fixed at 22pt would be
-    /// overrun by the glyph itself at a large system text size and the truncation rule would stop
-    /// being true. 1.7 em is the button plus its breathing room, measured in the same unit the glyph
-    /// grows in.
-    static func newCardButtonReserve(bodyPointSize: CGFloat) -> CGFloat {
-        em(1.7, bodyPointSize: bodyPointSize)
-    }
-
-    /// The room the header reserves for the **collapse chevron** beside the new-card button
-    /// (03-board-ui.md § Lane ▸ Collapsed lanes) — `newCardButtonReserve`'s twin, and it exists for
-    /// exactly that figure's reason: two glyphs now share the header's trailing edge, and a title has
-    /// to truncate before it reaches either.
-    ///
-    /// 1.4 em — 18pt at the standard 13pt body: the chevron is a smaller glyph than the plus and sits
-    /// closer to it than to the title, so it asks for less breathing room than the button it precedes.
+    /// This is the one figure that is not decoration: the chevron is an `Image` at
+    /// `.imageScale(.small)`, which *is* a relative size, so a reserve fixed at 18pt would be overrun
+    /// by the glyph itself at a large system text size and the truncation rule would stop being true.
+    /// 1.4 em — 18pt at the standard 13pt body — is the glyph plus its breathing room, measured in the
+    /// same unit it grows in.
     static func laneCollapseButtonReserve(bodyPointSize: CGFloat) -> CGFloat {
         em(1.4, bodyPointSize: bodyPointSize)
     }
 
-    /// Everything the lane header's trailing chrome takes — the chevron plus the new-card button. The
-    /// one figure the header pads by, so "a long title truncates before it collides" stays one rule
-    /// however many controls end up living there.
+    /// Everything the lane header's trailing chrome takes — the collapse chevron alone now that the
+    /// new-card button beside it is gone (this milestone narrowed this from a two-term sum to one).
+    /// Kept as its own figure rather than inlined at the call site: the one thing the header pads by,
+    /// so "a long title truncates before it collides" stays one rule however many controls end up
+    /// living there again.
     static func laneHeaderTrailingReserve(bodyPointSize: CGFloat) -> CGFloat {
-        newCardButtonReserve(bodyPointSize: bodyPointSize)
-            + laneCollapseButtonReserve(bodyPointSize: bodyPointSize)
+        laneCollapseButtonReserve(bodyPointSize: bodyPointSize)
     }
 
     /// The count badge's capsule inset — horizontal and vertical, which are deliberately different: