feat: integrate W axis as virtual A axis through gplan

Big-bang refactor of the W-axis integration. The auxcnc ESP stepper
is now exposed to the bbctrl planner (camotics gplan) as a virtual
A axis with no AVR motor mapping. gplan parses gcode for A natively,
applies soft limits, units, accel ramping and S-curve trajectories.
Line blocks with A motion are intercepted in Planner.__encode and
forked to the ESP via ExternalAxis on a worker thread; the residual
XYZ motion goes to the AVR as before.

This replaces the previous (MSG,HOOK:aux:N) side-channel: gcode
authors now write G1 A50 F1000 (or G28 A0 to home) and the planner
handles it the same way it handles X/Y/Z.

## Architecture

The AVR has 4 motor channels (0-3, all assigned to X/Y/Y/Z on
Onefinity). Looking at the AVR source, an axis with no motor
mapping is fully accepted: line blocks with that axis target update
ex.position[axis] in exec.c, but no motor steps because
motor_get_axis(motor)==axis returns -1. The AVR reports 'p' for
all 6 axes regardless. So we expose A to State as a synthetic
motor (index 4, host-only), populated from aux.json with full
kinematic config (vm/am/jm/tn/tm). State.find_motor and the
snapshot projection now walk 0..4. gplan sees A as a real axis.

## New module: ExternalAxis

  - Registers synthetic motor 4 with vm/am/jm/tn/tm so
    State.find_motor('a') returns 4 and gplan picks up
    soft limits + kinematics.
  - Worker thread drains a target queue so ESP RPCs (which can
    take seconds) never block the bbctrl ioloop.
  - execute_to_mm: synchronous, used by HTTP endpoints.
  - enqueue_target_mm: non-blocking, used by Planner.__encode.
  - home(): runs ESP cycle, syncs <axis>p and <axis>_homed.
  - abort(): drains queue.

## Planner

  - __encode splits external-axis target out of line blocks.
  - Pure A move -> emits id-sync only (planner advances cleanly).
  - Mixed XYZ + A -> AVR runs XYZ trapezoid concurrent with the
    ESP move (v1 accepts the slight desync; users wanting strict
    sequencing put A on its own gcode line).
  - _<axis>_homed for the synthetic motor mirrors into State only.
  - Planner.reset drains the worker queue and forces resync.

## Mach

  - Mach.home(axis='a') routes through ext.home() instead of the
    standard G28.2/G38.6 latch sequence (which doesn't apply to an
    ESP-driven axis), then issues G28.3 a<home> to sync gplan.
  - Mach.unhome strips the AVR path for A.
  - Mach.stop / E-stop drain the external-axis worker queue.
  - Mach.jog strips A so the AVR doesn't see it (continuous-rate
    jogging not supported on ESP yet; use /api/aux/jog instead).

## State

  - find_motor walks 0..4 (synthetic motor 4 lives in vars).
  - snapshot projection includes motor 4 so 4tn -> a_tn etc.
  - get_axis_vector picks up motor-4 values without changes.

## AuxAxis

  - Adds set_state_observer hook so ExternalAxis sees homed-flag
    changes after homing/boot-banner.
  - DEFAULTS now include axis_letter, max_velocity_m_per_min,
    max_accel_km_per_min2, max_jerk_km_per_min3 in user-facing
    motor-config units (m/min, km/min^2, km/min^3) matching the
    onefinity per-motor convention.

## AuxPreprocessor

  - Drops W-token rewriting entirely. M100..M103 ATC mapping kept.
  - W tokens in legacy gcode now warn (once per file) instead of
    being rewritten. Migration: replace W with A.

## Hooks

  - aux/aux_rel/aux_setzero hooks retired. aux_home kept as a
    legacy alias routing to ext.home() for older preprocessed
    gcode. ATC hooks (droptool/grabtool/release/clamp) unchanged.
  - E-stop now drains the external-axis worker queue.

## Web.py

  - /api/aux/{home,jog,move} now route through ExternalAxis when
    available so DRO and gplan position stay in sync.

## UI (axis-vars.js + control-view.pug)

  - _get_motor_id and _check_is_enabled fall back to motor index 4
    so the standard A column in the DRO renders state for the
    ESP-driven axis (with full offset / set-position / per-axis
    home support).
  - Legacy W row is gated on !a.enabled - shown only for installs
    that haven't migrated.
  - WAxisSettings.svelte exposes the new max_velocity_m_per_min /
    max_accel_km_per_min2 / max_jerk_km_per_min3 fields and an
    axis_letter selector for picking A/B/C.

## Open follow-ups (validate on hardware)

  - Q1: gplan soft-limit enforcement for A with min/max set.
    Easy smoke test: max_w=50, MDI G1 A100, expect rejection.
  - Q2: AVR behaviour with a target dict containing A values for
    a motorless axis. Read of exec.c suggests it's safe; needs a
    smoke test (no motor faults, no unexpected step counts).
  - Q3: pause/resume mid-A-move semantics. ESP doesn't honour
    bbctrl pauses; ext.abort drains the queue but a move-in-flight
    runs to completion. Acceptable for v1; v2 could add a synced
    pause.

src/pug/templates/control-view.pug

@@ -254,8 +254,13 @@ script#control-view-template(type="text/x-template")
                   @click=`home('${axis}')`)
                   .fa.fa-home
 
-          // W axis (auxiliary) — no offset, no set-zero / no set-position
-          .dro-row(:class="w.klass + ' ' + w.tklass", v-if="w.enabled",
+          // Legacy W axis row - shown only when the auxcnc stepper is
+          // *not* exposed as a virtual A axis. After v2 the standard
+          // A row above renders this axis natively (with full offset
+          // + set-position support); the W row stays for backwards
+          // compatibility with installs that haven't migrated.
+          .dro-row(:class="w.klass + ' ' + w.tklass",
+            v-if="w.enabled && !a.enabled",
             :title="w.title")
             .dro-axis.axis-w W
             .dro-pos: unit-value(:value="w.pos", precision=4)