Drive a Mach-O inspection end-to-end: alice (SwiftUI) attaches,
runs REPL probes, pins to notebook, shares the lab; bob (LumaGtk
via DYLD_INSERT'd frida-gadget) joins and verifies notebook
entries, sessions, REPL cells, and tracer events propagate.
Includes the observed.c fixture and a Frida agent for GTK widget
traversal.
