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/10-accessibility.md

@@ -10,7 +10,7 @@ The stance is committed in 00-vision.md: **accessibility is a requirement of "na
 
 ## The board through VoiceOver
 
-- **Tree shape**: window → lanes (accessibility containers, in lane `order`) → cards (leaf elements, in card `order`). A lane container is labeled "⟨title⟩, lane, N cards" — the count reads the search filter like the visible badge (04-interactions.md). The lane header's new-card button is a labeled child ("New card in ⟨lane⟩"). A card is **one flattened element**: label = title (or the untitled placeholder), value carries the attachment count when present, selected state via trait. Face icon and chips are decorative — folded into the element, never separately focusable: the flattened element carries the attachment count in its value, and the accessible attachment surface is the card window's keyboard-native section (below); the face itself has no media presentation (03-board-ui.md's no-carousel resettlement).
+- **Tree shape**: window → lanes (accessibility containers, in lane `order`) → cards (leaf elements, in card `order`). A lane container is labeled "⟨title⟩, lane, N cards" — the count reads the search filter like the visible badge (04-interactions.md). 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. Face icon and chips are decorative — folded into the element, never separately focusable: the flattened element carries the attachment count in its value, and the accessible attachment surface is the card window's keyboard-native section (below); the face itself has no media presentation (03-board-ui.md's no-carousel resettlement).
 - **Logical order, not masonry position** (decided): within a wide lane, VoiceOver reads cards by `order` — the interior grid columns are presentation only. This deliberately diverges from on-screen geometry (narrowed by the 2026-07-31 column-major masonry: walking down one column now *is* consecutive `order`; the divergence that remains is a geometry-sorted reading-order sweep — left-to-right, then down — which interleaves the columns); the spatial arrow-key model (04-interactions.md) remains available alongside, since board keyboard navigation keeps working with VoiceOver running.
 - **VO cursor and app selection are independent** (Finder-style): moving the VoiceOver cursor never mutates selection. VO-Space on any selectable element — card or lane header — toggles its selection (the ⌘-click analogue — a toggle, never plain click's replace, 04-interactions.md ▸ Selection; ruled 2026-07-29: a replace would silently wipe a multi-element selection, and one uniform VO-Space rule means the user never has to know the element kind to predict Space); ⌘↩ opens the card window; arrow keys and ⇧-arrows drive selection exactly as without VoiceOver. Selection state is always readable from the element (trait), and cut cards expose their dimmed pending state in the value ("cut, pending paste").
 - **Actions come from the context menu.** Context menus are the single inventory of per-item actions (Open, Rename, Delete, width stepper, …), reachable the standard way (VO-⇧-M); where SwiftUI additionally surfaces menu items as custom accessibility actions, that's free improvement, not a separate design surface. **The custom-action cut** (confirmed 2026-07-29): every plain button row of an item's context menu becomes a custom action; rows that open their own accessible surface (Style…'s popover) and non-action controls (the quick-style swatch picker) stay menu-only — the context menu remains the full inventory either way. Each action must call the same method as its menu row, so the two surfaces cannot drift.