feat: index runtime sessions by pane

docs/decisions/0004-runtime-actor-owns-each-pty.md

@@ -25,3 +25,9 @@ Dropping the UI-side actor disconnects its bounded channels, releases an actor
 blocked by event backpressure, terminates the process group through
 `lumbridge-pty`, and joins the actor thread. Tests use only synthetic shell
 fixtures and never provider credentials or real transcripts.
+
+A pane-indexed registry now owns multiple actors and routes commands and events
+without exposing pane identity to the actor itself. Duplicate ownership is
+rejected before spawning. Detaching a panel does not touch this registry;
+explicit registry shutdown is the process-terminating operation. The GPUI slice
+currently owns three registry entries, one for each terminal fixture.

docs/decisions/0009-responsive-ultrawide-work-lanes.md

@@ -20,7 +20,8 @@ resized through the runtime actor when the responsive panel count or window
 bounds change. On the 3440×1440 reference display the current fixed-cell spike
 derives approximately 71 columns by 42 rows per panel.
 
-The first actor slice still owns one real PTY and five deterministic comparison
-surfaces. Promoting the runtime boundary from one actor to a pane-indexed session
-registry is required before all five visible terminal panels can own independent
-real processes.
+The runtime slice now owns three independent real PTYs through a pane-indexed
+session registry, one for every terminal fixture. The Markdown, browser, and
+review fixtures remain deterministic comparison surfaces. Adding arbitrary new
+terminal panels requires replacing the fixed fixture identity pool with dynamic
+pane creation; it does not require another runtime ownership design.