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

KanbanUITests/AccessibilityVerification.md

@@ -81,7 +81,7 @@ Start with VoiceOver on (⌘F5), the scratch board frontmatter, and the trash hi
 
 > The new lane arrives with its title editor focused. Type `Later` and press Return. The lane now reads **"Later, lane, 0 cards"**.
 
-**2. Create a card.** With the new lane still active, ⌘N (File ▸ New Card). Alternatively VO-→ to the lane header's one child button, which reads **"New card in Later" [button]**, and VO-Space it.
+**2. Create a card.** With the new lane still active, ⌘N (File ▸ New Card).
 
 > The card placeholder appears with its editor focused. Type `Smoke test` and press Return. VO-→ onto it: **"Smoke test"**. No value is spoken — it has no attachments and is not cut.
 
@@ -164,7 +164,7 @@ The pane ships with the comments feature; 10-accessibility.md ▸ Comments is th
 - [ ] **Increase Contrast** (Accessibility ▸ Display ▸ Increase contrast): strokes gain a flat point, borderless card and lane plates gain a resting separator hairline, faded accents go to full alpha, and the selection ring and plate edge both strengthen. Hierarchy must be preserved — everything strengthening by the same amount is the intent, not a bug.
 - [ ] **Reduce Transparency**: the transient search bar's glass goes solid, and so do the alpha washes that composite over a user-chosen board background — the trash plate, the hatched trash header, the drag shadow. Look for any remaining see-through surface.
 - [ ] **Reduce Motion**: movement animations go instant, appear/disappear transitions go crossfade — uniformly. Named cases to walk: reflow-on-drag, search animate-out, the drag replica's lift and settle, the lane-resize rubber band, the trash column's appear/disappear, and the store's reload seam (the largest animated surface in the app). The drag replica's 1:1 tracking and the selection marquee correctly have no reduced variant.
-- [ ] **Full Keyboard Access** (Keyboard ▸ Keyboard navigation, VoiceOver **off**): the board is **one** tab stop, not one per card, and its focus ring becomes visible under FKA when it is hidden without. Tab must reach every lane's new-card button, the toolbar, the search field, the board popover's controls, the card window, and welcome.
+- [ ] **Full Keyboard Access** (Keyboard ▸ Keyboard navigation, VoiceOver **off**): the board is **one** tab stop, not one per card, and its focus ring becomes visible under FKA when it is hidden without. Tab must reach every lane's collapse chevron, the toolbar, the search field, the board popover's controls, the card window, and welcome.
 - [ ] **The template chooser is arrow-navigable** under FKA: tiles take focus, arrows move between them and clamp at the ends, Space picks, Return is still the sheet's default action, and focus follows the selection one way only.
 - [ ] **The style editor's grids** are arrow-navigable, every well is Tab-reachable and labeled by name (palette color, symbol name; leading wells "None" / "Default"), the current value is stated by trait, and a batch selection's mixed state reads as "mixed".
 - [ ] **State is never colour-alone**, everywhere: selection is ring plus trait, cut-pending is dim plus stated value, the trash header is hatched plus labeled.