feat: index runtime sessions by pane

docs/TESTING.md

@@ -111,9 +111,10 @@ who already have the final cargo-watch release installed.
   input, resize, one-chunk backpressure, hung-process termination, invalid
   dimensions, and exclusion of provider credentials from the child environment.
 - `lumbridge-runtime` tests ordered actor output, serialized input and resize,
-  event polling, invalid configuration, and bounded-time cleanup of a hung PTY.
-  A cross-crate integration test drives a real synthetic shell through actor
-  output, VT parsing, terminal key encoding, and a 41×101 resize.
+  event polling, invalid configuration, bounded-time cleanup of a hung PTY,
+  pane-keyed input/output isolation, duplicate ownership, and independent
+  shutdown. A cross-crate integration test drives a real synthetic shell through
+  actor output, VT parsing, terminal key encoding, and a 41×101 resize.
 - `lumbridge-terminal` fixtures cover styled Unicode, cursor/title state,
   alternate screen, bracketed paste, protocol replies, sanitized title and
   blocked OSC 52 behavior, key encoding, application-cursor mode, and resize.
@@ -123,8 +124,9 @@ who already have the final cargo-watch release installed.
 - The GPUI slice tests its key-event adapter, responsive one/three/five-panel
   geometry, selected-panel windowing, per-panel 20/60/20 PTY sizing, xterm
   256-color conversion, styled-run coalescing, cursor-run boundaries, and
-  scrollback shortcut routing. It renders one real actor-owned VT session while
-  five surfaces continue their deterministic background workload.
+  scrollback shortcut routing. It renders three independently actor-owned VT
+  sessions while three non-terminal surfaces continue their deterministic
+  background workload.
 - `lumbridge-terminal` retains 10,000 history lines by default and tests
   framework-neutral page/top/bottom viewport movement, revision changes, and
   live-bottom no-ops without sending history-navigation bytes to the child PTY.