muxg: prediction overlay

open   by a73x

The TUI paints typed bytes ahead of the daemon's echo through `src/tui/predict.zig` (an overlay that never enters the replica). `muxg` paints only what the daemon sent, so over a WAN or through ssh it feels slower than the TUI on the same link. Once the wall retires, the GUI is the multi-session surface for exactly those remote boxes.

Wanted: the prediction overlay behind the GUI's replica, sharing the predictor's state machine (`predict.zig` currently sits under `src/tui/`; moving the state machine to a client-side module and keeping the two painters separate keeps rule 4 and the "overlay never enters the replica" invariant). A gate on retiring the wall.