feat: index runtime sessions by pane

docs/ARCHITECTURE.md

@@ -65,16 +65,18 @@ clipboard writes are disabled by default. The renderer never imports the
 upstream terminal type, and protocol replies return through the same actor input
 queue as human input. See decision 0005.
 
-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 feeds one actor session
-through the terminal engine while five surfaces retain deterministic comparison
-output. Its adapter groups adjacent cells into native GPUI paint runs and renders
+Each `lumbridge-runtime` actor owns one `PtySession` on a dedicated thread.
+Bounded command and event queues serialize input, resize, close, and shutdown
+against ordered raw-byte output. A generic pane-indexed registry now owns and
+routes multiple actors without knowing whether their panels are attached. The
+GPUI slice feeds three independent actor sessions through three terminal engines
+while three non-terminal surfaces retain deterministic comparison output. Its
+adapter groups adjacent cells into native GPUI paint runs and renders
 ANSI/indexed/RGB colors, emphasis, hyperlinks, and cursor shapes. Window geometry
 drives terminal rows and columns and resizes both the engine and PTY. The engine
 now exposes retained-history offsets and page/top/bottom viewport movement
 without writing scroll keys to the PTY. Text selection, mouse reporting, and a
-lower-level terminal canvas remain. This actor still runs in-process; moving the
+lower-level terminal canvas remain. These actors still run in-process; moving the
 same framework-neutral contract behind local authenticated IPC is the next
 durability step. See decisions 0004 and 0005.