feat(services): open-core community/professional tier split

Pro-tier honeypots load from an optional decnet/services/pro/ subpackage that
the registry auto-discovers when present; the Community build omits it, so the
directory's absence IS the entitlement gate (no runtime licence check). Recurse
subclasses so a pro service may extend a community one. Exclude pro from the
community wheel and git-ignore the path (it lives in the private
decnet-professional repo).

Add LICENSING.md documenting the dual-license: AGPL-3.0-or-later core plus a
commercial EULA for the Professional tier.

decnet/services/registry.py

@@ -5,6 +5,12 @@ Service plugin registry.
 Auto-discovers all BaseService subclasses by importing every module in the
 services package. Adding a new service requires nothing beyond dropping a
 new .py file here that subclasses BaseService.
+
+Professional-tier honeypots live in the optional ``decnet.services.pro``
+subpackage, which ships only in the Professional build (a private tree merged
+in at packaging time) and is absent from the open-core Community build. The
+registry scans it when present, so absence of the directory IS the entitlement
+gate — no licence check, no feature flag.
 """
 
 import importlib
@@ -17,6 +23,13 @@ _registry: dict[str, BaseService] = {}
 _loaded = False
 
 
+def _all_subclasses(cls: type) -> set[type]:
+    # Recurse: a pro honeypot may extend a community service, not BaseService
+    # directly, and __subclasses__() only returns direct children.
+    subs = set(cls.__subclasses__())
+    return subs.union(*(_all_subclasses(s) for s in subs))
+
+
 def _load_plugins() -> None:
     global _loaded
     if _loaded:
@@ -26,7 +39,12 @@ def _load_plugins() -> None:
         if module_info.name in ("base", "registry"):
             continue
         importlib.import_module(f"decnet.services.{module_info.name}")
-    for cls in BaseService.__subclasses__():
+    # Professional build only: present == entitled. Community build has no pro/.
+    pro_dir = package_dir / "pro"
+    if pro_dir.is_dir():
+        for mi in pkgutil.iter_modules([str(pro_dir)]):
+            importlib.import_module(f"decnet.services.pro.{mi.name}")
+    for cls in _all_subclasses(BaseService):
         if not cls.__module__.startswith("decnet.services."):
             continue
         instance = cls()  # type: ignore[abstract]