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/FileDropWriteTests.swift

@@ -432,8 +432,8 @@ struct CreateCardsFromFilesTests {
     }
 
     /// **The create half is a user-initiated creation, so it clears the query** (04-interactions.md
-    /// § Search, stated by mechanism: "⌘N, Return-creation, the header button, empty-space
-    /// double-click, paste, and Finder file drops alike"). The card is titled `shot`, which the
+    /// § Search, stated by mechanism: "⌘N, Return-creation, empty-space double-click, paste, and
+    /// Finder file drops alike"). The card is titled `shot`, which the
     /// standing query would hide — the whole point of the carve-out.
     @Test("A file drop that creates cards clears the search — the same rule ⌘N obeys")
     func creatingFromFilesClearsTheSearch() throws {