Planner: lazy-load camotics.gplan so HTTP listener comes up first

Importing camotics.gplan pulls in a C++ extension (libstdc++,
boost::python, etc.) which adds several seconds to bbctrl startup
on the Pi. Defer it to Planner.init() — bbctrl can serve the UI
and accept connections without ever touching the planner, and the
penalty is paid only the first time motion is queued.

src/py/bbctrl/Planner.py

@@ -30,7 +30,22 @@ import math
 import re
 import time
 from collections import deque
-import camotics.gplan as gplan # pylint: disable=no-name-in-module,import-error
+# camotics.gplan is heavy (loads a C++ extension that pulls in libstdc++,
+# boost::python, etc.). Defer it: bbctrl can listen on HTTP and serve
+# the UI without ever touching the planner. Lazy-load the first time
+# Planner.init() runs, which is when the user actually queues motion.
+gplan = None
+def _load_gplan():
+    global gplan
+    if gplan is None:
+        try:
+            import bbctrl.Trace as _T
+            with _T.span('imports.camotics_gplan'):
+                import camotics.gplan as _gplan # pylint: disable=no-name-in-module,import-error
+        except Exception:
+            import camotics.gplan as _gplan # pylint: disable=no-name-in-module,import-error
+        gplan = _gplan
+    return gplan
 import bbctrl.Cmd as Cmd
 from bbctrl.CommandQueue import CommandQueue
 
@@ -329,7 +344,7 @@ class Planner():
         if stop:
             self.ctrl.mach.stop()
 
-        self.planner = gplan.Planner()
+        self.planner = _load_gplan().Planner()
         self.planner.set_resolver(self._get_var_cb)
         # TODO logger is global and will not work correctly in demo mode
         self.planner.set_logger(self._log_cb, 1, 'LinePlanner:3')