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

DESIGN/11-command-nexus.md

@@ -90,7 +90,7 @@ All board grammar keys are inert while a title editor is focused, and menu dispa
 - **Lane edge drag** (03 ▸ Lane): window-growing resize between integer widths — the one width control that moves the window.
 - **Finder file drops** (04): onto a card = attach; onto lane empty space = one card per file; anywhere on the card window = attach (05's payload-split precedence).
 - **Preview** (05): task-list checkbox toggle (the one interactive exception), link opens, text selection. **In-content controls aren't P-only** (settled): checkboxes and links are real controls in the focus/accessibility tree, so Full Keyboard Access + Space and VO-Space reach them (05 ▸ Task-list checkboxes) — content rides the system focus model rather than earning command rows; "a command absent here doesn't exist" scopes to commands, not content.
-- **Lane header new-card button** (03, labeled "New card in ⟨lane⟩" — 10): creates in **the button's lane**, appended at the bottom — the click names its target, overriding 04's selection-derived ⌘N target rule (settled); placeholder and abandon semantics exactly as ⌘N. The card window's attachments-section **quiet add affordance** (05) is the same class: a pointer twin of File ▸ Add Attachment…, no separate behavior.
+- **The card window's attachments-section quiet add affordance** (05): a pointer twin of File ▸ Add Attachment…, no separate behavior. (The lane header once carried a matching "+" new-card button with its own click-names-target carve-out over 04's ⌘N rule; it is removed — every creation path now goes through the selection-derived rule, Return on a selected lane, or empty-space double-click.)
 - **Attachment rows** drag out their file URL (05). **Welcome rows**: single click selects, double click opens (03).
 
 ## Context menus