docs: A axis architecture (renamed from W) + README section

- Move docs/AUX_W_AXIS.md to docs/AUX_A_AXIS.md and rebadge W -> A
  throughout, with a header note pointing at ExternalAxis as the
  current implementation.
- README: A-axis fork heading, link to AUX_A_AXIS.md, /api/aux/status
  in verify-flash, small comment in scripts/deploy/local.sh.

scripts/deploy/local.sh

@@ -9,6 +9,9 @@
 #   * The full V09 chrome (header tabs, settings rail, jog grid, DRO
 #     skeleton, status strip).
 #   * A "DISCONNECTED" overlay because there's no controller backend.
+#   * The A axis row in jog/DRO is hidden (correct: it appears only when
+#     the controller reports `aux_enabled = true`). To exercise the A
+#     axis end-to-end, deploy to the Pi (`./deploy.sh hardware`).
 
 set -euo pipefail