docs(split): comments say what TerminalTile does now

The rename from SplitTerminalPane carried comments over that the PR 1
changes made untrue:

- constants.js buildSplitPickerSessions said TerminalTile._sendResize has
  no detached check (it stands aside like the primary pane) and that Pane
  B sends no `seq` (its input rides the exactly-once queue). The
  conclusions stay: a detached session's window owns its PTY size, and a
  session with no PTY has a pane nothing feeds or reads.
- terminal-split.js said TerminalTile has no "dims unchanged" skip (it has
  _lastSentDims), and its @loadorder still ended at respawn-ui.js rather
  than tile-grid.js.
- terminal-tile.js still called every pane "Pane B", said a shell load
  lands in "a 50000-line xterm" (the scrollback is an option now) and
  that a TUI session always gets a full replay (with boundedLoad, grid
  tiles get the bounded window), and told some reasons as history ("an earlier draft", "used
  to", "It LOOKED intermittent"). Those now give the reason in the
  present tense, and the comments touched lose their em dashes.
  writeChunked's doc comment is left as it is: the perf work rewrites
  that function and owns its comment.

Comments only.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/constants.js

@@ -1612,21 +1612,18 @@ function buildSplitPickerSessions(sessions, sessionOrder, excludeId, detachedIds
   const result = [];
   for (const id of sessionOrder) {
     if (id === excludeId) continue;
-    // A detached (popped-out) session's own window already yields its PTY
-    // size (see sendResize's detachedElsewhere guard in terminal-ui.js) —
-    // Pane B's TerminalTile._sendResize() has no such check, so letting
-    // one into the picker put its detached window and Pane B in a fight over
-    // the same PTY's dimensions.
+    // A detached (popped-out) session's own window owns its PTY size (see
+    // sendResize's detachedElsewhere guard in terminal-ui.js;
+    // TerminalTile._sendResize() stands aside the same way), so Pane B could
+    // only show it at a size it cannot set.
     if (detachedIds?.has?.(id)) continue;
     const session = sessions.get(id);
     if (!session) continue;
     // A session with no PTY attached (exited CLI, a crash-looped session
     // whose breaker tripped, a restore that failed to re-attach) has nothing
-    // reading its tmux pane. TerminalTile never does selectSession()'s
-    // re-attach POST, so its socket would open onto a pane nothing feeds:
-    // no terminal events, and Session.write() silently drops every keystroke
-    // with no ack either way (Pane B sends no `seq`), so the loss is
-    // invisible — the healthy socket never trips the disconnect banner.
+    // reading its tmux pane, and the split never does selectSession()'s
+    // re-attach POST: Pane B would open a healthy-looking socket onto a pane
+    // that nothing feeds and nothing reads.
     if (session.pid === null) continue;
     result.push({ id, label: session.name || 'Session' });
   }