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.

tests/hermes_cli/test_pty_bridge.py

@@ -8,7 +8,6 @@ from __future__ import annotations
 
 import os
 import shutil
-import signal
 import sys
 import time
 
@@ -212,75 +211,6 @@ class TestPtyBridgeClose:
                 break
         assert reaped, f"pid {pid} still running after close()"
 
-    def test_close_signals_child_process_group(self, monkeypatch):
-        sent: list[tuple[int, signal.Signals]] = []
-
-        class _FakeProc:
-            pid = 12345
-            fd = -1
-
-            def __init__(self):
-                self.alive = True
-
-            def isalive(self):
-                return self.alive
-
-            def kill(self, sig):
-                raise AssertionError(f"single-process kill used: {sig}")
-
-            def close(self, force=False):
-                self.closed = force
-
-        fake = _FakeProc()
-
-        def fake_killpg(pgid, sig):
-            sent.append((pgid, sig))
-            fake.alive = False
-
-        monkeypatch.setattr(os, "getpgid", lambda pid: 67890)
-        monkeypatch.setattr(os, "killpg", fake_killpg)
-
-        bridge = PtyBridge.__new__(PtyBridge)
-        bridge._proc = fake
-        bridge._fd = -1
-        bridge._closed = False
-
-        bridge.close()
-
-        assert sent == [(67890, signal.SIGHUP)]
-        assert bridge._closed is True
-
-    def test_close_falls_back_to_single_process_signal_when_group_unknown(self, monkeypatch):
-        sent: list[signal.Signals] = []
-
-        class _FakeProc:
-            pid = 12345
-            fd = -1
-
-            def __init__(self):
-                self.alive = True
-
-            def isalive(self):
-                return self.alive
-
-            def kill(self, sig):
-                sent.append(sig)
-                self.alive = False
-
-            def close(self, force=False):
-                self.closed = force
-
-        monkeypatch.setattr(os, "getpgid", lambda pid: (_ for _ in ()).throw(OSError()))
-
-        bridge = PtyBridge.__new__(PtyBridge)
-        bridge._proc = _FakeProc()
-        bridge._fd = -1
-        bridge._closed = False
-
-        bridge.close()
-
-        assert sent == [signal.SIGHUP]
-
 
 @skip_on_windows
 class TestPtyBridgeEnv: