docs: Pane B is a TerminalTile (reconnect, exactly-once input, focus)

CLAUDE.md, architecture-invariants#split-pane-sessions and the split-pane
spec described Pane B as having no reconnect, seq-less input and xterm's
own Ctrl+V. Updated for TerminalTile (terminal-tile.js, load order 7.4):
reconnect and stop codes, the input-socket map and which input is kept
out of the persisted queue, image paste, the geometry rules, and
_focusedPane() with its Ctrl+W exception. The tile-grid spec now records
PR 1 as built (no key handler factory; scheduleLoad and scrollback move
to PR 2 with their first user).

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

docs/split-pane-sessions-plan.md

@@ -4,6 +4,13 @@
 **Author**: Claude (session with Tim), 2026-09-15
 **Scope**: v1 only. v2 items are named and explicitly deferred, not designed.
 
+> **Update (tile grid, PR 1):** Pane B is now a `TerminalTile`
+> (`terminal-tile.js`) and is no longer as plain as this spec describes: it
+> reconnects after a drop, delivers input exactly once, has clickable paths
+> and image paste, sizes its PTY without a floor and adopts `zc` columns, and
+> the app-level terminal shortcuts follow the focused pane (Ctrl+W excepted).
+> See `docs/tile-grid-plan.md` and `architecture-invariants#split-pane-sessions`.
+
 ## Problem
 
 Codeman's terminal area shows exactly one active session (pane) at a time —