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.

README.md

@@ -1,8 +1,9 @@
-# OneFinity CNC Controller Firmware (community fork)
+# OneFinity CNC Controller Firmware (A-axis fork)
 
-This is the OneFinity / Buildbotics bbctrl firmware with a redesigned
-UI (V09), Font Awesome 6, faster cold boot, and a streamlined macOS
-dev / deploy workflow.
+This is the community-fork firmware (V09 UI, FA6, cold-boot work,
+macOS dev tooling) with a virtual A axis driven by an auxcnc ESP32
+over USB serial. See [docs/AUX_A_AXIS.md](docs/AUX_A_AXIS.md) for the
+design and config.
 
 ## Layout
 
@@ -16,7 +17,7 @@ src/svelte-components/  Newer Svelte UI for dialogs and settings
 src/pug/        Pug templates compiled into build/http/index.html
 src/resources/  Static assets and config templates
 scripts/        Install / update / RPi build helpers
-docs/           Architecture, dev setup
+docs/           Architecture, dev setup, A-axis docs
 ```
 
 ## Build & flash (quick path, macOS or Linux)
@@ -101,6 +102,7 @@ bbctrl restarts, then the new UI).
 ```bash
 curl -s http://onefinity.local/ | grep -c "OneFinity"
 curl -s http://onefinity.local/api/diag/timing | head
+curl -s http://onefinity.local/api/aux/status   # if A axis is enabled
 ```
 
 ## Build & flash (full path, Debian/Linux)
@@ -108,3 +110,15 @@ curl -s http://onefinity.local/api/diag/timing | head
 For AVR + GPlan rebuilds, see [docs/development.md](docs/development.md).
 That path uses qemu + chroot to cross-compile gplan for ARM and needs
 the `gcc-avr` / `avr-libc` toolchain.
+
+## A axis (auxcnc)
+
+This fork adds a virtual A axis. See
+[docs/AUX_A_AXIS.md](docs/AUX_A_AXIS.md) for:
+
+- G-code surface (`G28 A0`, `G1 A25`, etc.)
+- The G-code preprocessor and hook architecture
+- aux.json keys
+- REST API (`/api/aux/*`)
+- UI surface (jog row in Control, settings panel in Settings)
+- Edge cases (ESP reboot mid-job, limit closed at home start, …)