Hooks v2: block unpause until hook completes

- Blocking hooks (block_unpause: true, default for tool-change) run
  in a background thread and gate Mach.unpause() via can_unpause()
- Machine stays in HOLDING state while hook runs — AVR steppers idle,
  spindle state preserved, position locked
- auto_resume option to unpause automatically after hook completes
- E-stop cancels any running hook immediately
- Hook status pushed to frontend via state (hook_busy, hook_event)
- GET /api/hooks/status endpoint for polling
- Non-blocking hooks (program-start, program-end, etc.) fire-and-forget

src/py/bbctrl/Web.py

@@ -776,6 +776,11 @@ class HooksSaveHandler(bbctrl.APIHandler):
         self.get_ctrl().hooks.save_config(self.json)
 
 
+class HooksStatusHandler(bbctrl.APIHandler):
+    def get(self):
+        self.write_json(self.get_ctrl().hooks.get_status())
+
+
 class HooksFireHandler(bbctrl.APIHandler):
     def put_ok(self, event):
         data = self.json if hasattr(self, 'json') and self.json else {}
@@ -959,6 +964,7 @@ class Web(tornado.web.Application):
             (r'/api/remote-diagnostics', RemoteDiagnosticsHandler),
             (r'/api/hooks', HooksGetHandler),
             (r'/api/hooks/save', HooksSaveHandler),
+            (r'/api/hooks/status', HooksStatusHandler),
             (r'/api/hooks/fire/([\w-]+)', HooksFireHandler),
             (r'/(.*)', StaticFileHandler,
              {'path': bbctrl.get_resource('http/'),