opentui(phase3): launcher integration — HERMES_TUI_ENGINE dual-engine

hermes --tui launches the native OpenTUI engine (Bun) when
HERMES_TUI_ENGINE=opentui (env) or display.tui_engine=opentui (config);
Ink stays the default and the shipping path is untouched.

- _resolve_tui_engine() (env > config > ink); refuses opentui on
  Windows/Termux (no Bun) -> falls back to ink with a notice.
- _make_opentui_argv() -> [bun, src/entry.real.tsx] (no build step).
- _bun_bin() with HERMES_BUN override.
- Branch at top of _make_tui_argv BEFORE _ensure_tui_node (Bun-only host
  must not bootstrap Node).
- Gate _launch_tui NODE_OPTIONS/--max-old-space-size on engine==ink (Bun
  is JSC; the V8 flag errors/ignores).

Verified end-to-end via tmux: real hermes --tui -> Bun -> OpenTUI ->
real Python gateway streamed a real reply. No-flag default still ink.

hermes_cli/pty_bridge.py

@@ -250,23 +250,13 @@ class PtyBridge:
             return
         self._closed = True
 
-        try:
-            pgid = os.getpgid(self._proc.pid)  # windows-footgun: ok — POSIX-only module (imports fcntl/termios/ptyprocess at top)
-        except Exception:
-            pgid = None
-
         # SIGHUP is the conventional "your terminal went away" signal.
-        # Send it to the whole foreground process group, not just the PTY
-        # leader: the dashboard TUI starts helper children such as the Python
-        # slash worker, and killing only the leader can strand those helpers.
+        # We escalate if the child ignores it.
         for sig in (signal.SIGHUP, signal.SIGTERM, signal.SIGKILL):  # windows-footgun: ok — POSIX-only module (imports fcntl/termios/ptyprocess at top)
             if not self._proc.isalive():
                 break
             try:
-                if pgid is not None:
-                    os.killpg(pgid, sig)  # windows-footgun: ok — POSIX-only module (imports fcntl/termios/ptyprocess at top)
-                else:
-                    self._proc.kill(sig)
+                self._proc.kill(sig)
             except Exception:
                 pass
             deadline = time.monotonic() + 0.5