Spawn TUI panes in slosh, the multiplexer we actually use

`pane spawn` knew tmux and zellij, so under slosh every entry point
answered "error: not inside tmux or zellij" — the agent skill described a
workflow that could not run. slosh is now the first thing autodetected
($SLOSH, then $ZELLIJ, then $TMUX), and a slosh pane id is a bare integer,
so a registry row written before this commit still routes its own
stop/capture/keys.

Everything goes through the control socket, always the JSON form
($SLOSH_BIN -s $SLOSH_SESSION cmd): the bare-verb alias answers unwrapped,
and a parser written for one shape silently finds nothing in the other.
$SLOSH_BIN rather than `slosh` because a session may have been started
from a build tree that is not on $PATH.

Spawning is the one place slosh differs in kind. Its `split` verb only
ever makes a shell pane, and typing a command into somebody's shell is
the thing not to do — a pane that was *given* a command is the one with
alive/exit_code to poll and `rerun` to repeat. So spawn applies a one-tab
layout whose pane carries command= and a declared purpose=idatui:<hex>,
finds it by that purpose (declared purposes are locked, so the TUI cannot
relabel itself out from under us, and titles change anyway), then
move-panes it into the tab we were called from: it lands beside the
caller with its pty and scrollback intact, and the layout's now-empty tab
removes itself. --detached restores focus afterwards, since slosh focuses
what it creates and moves.

The rest follows the zellij precedent. A dead command pane stays on
screen as a husk, which _pane_alive must read as dead or a launcher that
died on startup would burn the whole 300s readiness timeout instead of
reporting "pane exited during startup". `capture` cuts the pane's
content_* rect out of `snapshot` (which is the whole composited session)
and hops tabs if the pane is parked in another one, because only the
active tab is composed at all. `keys` keeps taking tmux-flavoured names
and turns them into the bytes a terminal sends: it focuses the target,
writes with `raw`, and restores focus — `send` would be decoded as
session input first, where C-a is slosh's own leader and never reaches
the TUI. --size stays tmux-only and now says so for whichever mux is in
play.

The two RPC integration tests gated on $TMUX/$ZELLIJ, so they skipped
silently under slosh; they accept $SLOSH now.

Verified against the live session, not just in unit checks: a probe pane
spawns, reports alive, captures its own output, receives real keystrokes
and dies on close; `pane spawn --open targets/echo` reaches ready:true
with 128 functions, `drive where` answers, `pane keys Tab` flips listing
to pseudocode on screen, and `pane stop` leaves an empty registry and no
husk. Env inheritance is unchanged from zellij (a pane gets the session
server's environment, not the caller's) — the probe printed PROBE_VAR=[].

1 files changed, 1 insertions, 1 deletions

diff --git a/tools/demo.py b/tools/demo.py
index 5cf5207..404f137 100644
--- a/tools/demo.py
+++ b/tools/demo.py
@@ -9,7 +9,7 @@ being used.
     # render the TUI in THIS terminal and drive it (best for a single-pane capture)
     python tools/demo.py --here
 
-    # or give it its own pane in tmux/zellij
+    # or give it its own pane in slosh/tmux
     python tools/demo.py --spawn
 
     # or record a session you set up yourself (your pane, your size, your zoom)