Config: idempotent macro file rename W -> A

The auxiliary stepper used to be exposed as a W axis. After the
gplan integration it is exposed as A. Migrate persisted macro
config on every load:
  w_down.nc -> a_down.nc
  w_up.nc   -> a_up.nc
  'W Down'  -> 'A Down'
  'W Up'    -> 'A Up'

Idempotent so a stale in-memory copy can never reintroduce the old
names.

src/py/bbctrl/Config.py

@@ -216,6 +216,32 @@ class Config(object):
                 defaults = json.load(f)
                 config['selected-tool-settings'] = defaults['selected-tool-settings'];
 
+        # Auxiliary axis nomenclature: rename W -> A in macro names and
+        # filenames. The auxcnc-driven stepper has been integrated into
+        # gplan as A since the option-b migration; old configs may
+        # still carry W Down/W Up macro entries pointing at
+        # w_down.nc/w_up.nc which were renamed on disk to a_down.nc /
+        # a_up.nc. Migrate idempotently on every load so a stale
+        # in-memory copy can never reintroduce the old names.
+        macros = config.get('macros') if isinstance(config, dict) else None
+        if isinstance(macros, list):
+            renames = {
+                'w_down.nc': 'a_down.nc',
+                'w_up.nc':   'a_up.nc',
+            }
+            display_renames = {
+                'W Down': 'A Down',
+                'W Up':   'A Up',
+            }
+            for m in macros:
+                if not isinstance(m, dict): continue
+                fn = m.get('file_name')
+                if isinstance(fn, str) and fn in renames:
+                    m['file_name'] = renames[fn]
+                nm = m.get('name')
+                if isinstance(nm, str) and nm in display_renames:
+                    m['name'] = display_renames[nm]
+
         config['version'] = self.version.split('b')[0]
         config['full_version'] = self.version