fix(daemon)+fix(client): close four fool-proofing gaps

C++ (equipment_service.hpp):
- ReportSubstrate ARRIVED: check CreateResult and return INVALID_OBJECT
  when the substrate ID already exists, instead of silently doing nothing
- ReportSubstrate/ReportModule default switch branches: return false
  (→ CANNOT_DO_NOW) for unknown enum values instead of silently accepting

Python (_client.py):
- @eq.command: raise NameError (client-side name validation) instead of
  SecsGemError (which means "daemon declined a request") — wrong type
- Module docstring: update example to show @eq.command / eq.names API

Test (test_daemon_service.cpp):
- Add duplicate-ARRIVED check (expects INVALID_OBJECT)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

clients/python/secsgem_client/_client.py

@@ -10,11 +10,12 @@ equipment.yaml; values are plain Python.
 
     eq = Equipment("localhost:50051")
     eq.set(ChamberPressure=2.5)
-    eq.fire("ProcessStarted")
+    eq.fire(eq.names.event.ProcessStarted)   # typo-safe; plain strings also work
 
-    @eq.on("START")
-    def start(cmd):
+    @eq.command                              # function name IS the RCMD name,
+    def START(cmd):                          # validated against equipment at import
         run_recipe(cmd.params.get("PPID"))
+        eq.fire(eq.names.event.ProcessStarted)
 
     eq.listen()   # blocks, dispatching host commands to your handlers
 """
@@ -238,9 +239,7 @@ class Equipment:
         if name not in self.names.command:
             close = difflib.get_close_matches(name, dir(self.names.command), n=3)
             hint = f" Did you mean {', '.join(close)}?" if close else ""
-            raise SecsGemError(
-                pb.Ack.PARAMETER_INVALID,
-                f"no host command '{name}' to bind @eq.command to.{hint}")
+            raise NameError(f"no host command '{name}' to bind @eq.command to.{hint}")
         self._handlers[name] = fn
         return fn