Add bounded runtime actor and live PTY pane

docs/ARCHITECTURE.md

@@ -41,8 +41,8 @@ messages should be real from the beginning.
 - `lumbridge`: installable application entry point.
 
 The scaffold currently contains `lumbridge-core`, `lumbridge-storage`,
-`lumbridge-buzz`, `lumbridge-pty`, and the entry point. Larger runtime crates are
-added after their architecture spikes pass.
+`lumbridge-buzz`, `lumbridge-pty`, `lumbridge-runtime`, and the entry point.
+Larger runtime crates are added after their architecture spikes pass.
 
 ## Terminal path
 
@@ -55,6 +55,13 @@ scrollback. The terminal engine turns output into immutable render snapshots and
 bounded deltas for the UI. Scrollback is chunked and persisted separately from
 the live screen to prevent large agent transcripts from blocking input.
 
+The first `lumbridge-runtime` actor now owns one `PtySession` on a dedicated
+thread. Bounded command and event queues serialize input, resize, close, and
+shutdown against ordered raw-byte output. The GPUI slice consumes one actor
+session while five surfaces retain deterministic comparison output. This actor
+still runs in-process; moving the same framework-neutral contract behind local
+authenticated IPC is the next durability step. See decision 0004.
+
 We should evaluate, not blindly copy, WezTerm, Zellij, RMUX, tmux, and cmux. The
 first spike must compare a reusable terminal crate with a small first-party layer.
 Correctness cases include alternate screen, bracketed paste, OSC 8 links, Kitty