Hooks: ATC IPC layer between gcode preprocessor and runtime

Adds bbctrl.Hooks: a small dispatch layer for HOOK:<event>:<data>
messages embedded in g-code as (MSG,HOOK:droptool:) etc. Hooks can
block the unpause until the registered callback completes and
auto-resume after.

- bbctrl.Hooks: registry, fire, dispatch_hook_message, persistent
  config in hooks.json, REST surface (/api/hooks, /api/hooks/save,
  /api/hooks/status, /api/hooks/fire/<event>).
- Ctrl: instantiate self.hooks alongside the other subsystems.
- Planner._add_message: when a (MSG,...) line is HOOK:<event>:<data>,
  route it through ctrl.hooks instead of state.messages so it never
  surfaces as a UI popup and dispatch is immediate (state.messages
  has a 250ms debounce).
- Web: handlers for the /api/hooks routes.

src/py/bbctrl/Ctrl.py

@@ -71,6 +71,8 @@ class Ctrl(object):
                     self.jog = bbctrl.Jog(self)
             with Trace.span('ctrl.pwr'):
                 self.pwr = bbctrl.Pwr(self)
+            with Trace.span('ctrl.hooks'):
+                self.hooks = bbctrl.Hooks(self)
 
             with Trace.span('ctrl.mach.connect'):
                 self.mach.connect()