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/gateway/test_simplex_plugin.py

@@ -205,13 +205,6 @@ def test_corr_id_pending_set_self_trims():
 
 @pytest.mark.asyncio
 async def test_send_dm():
-    """DMs use the bare ``@<id> text`` chat-command form.
-
-    The bracketed form ``@[<id>] text`` is what the daemon's man page
-    documents, but in practice both addressing styles route through
-    the same chat-command parser; bare ``@<id>`` matches what every
-    Hermes deployment has been using in production for months.
-    """
     from gateway.config import PlatformConfig
     cfg = PlatformConfig(enabled=True, extra={"ws_url": "ws://localhost:5225"})
     adapter = SimplexAdapter(cfg)
@@ -229,14 +222,6 @@ async def test_send_dm():
 
 @pytest.mark.asyncio
 async def test_send_group():
-    """Groups use the structured ``/_send #<id> json [...]`` form.
-
-    The bracket chat-command form ``#[<id>] text`` *looks* like an exact
-    ID match in the daemon docs but is parsed as a display-name lookup
-    — so messages to groups whose display name isn't literally the ID
-    silently drop. The structured ``/_send`` form addresses by numeric
-    ID and survives newlines/quoting through ``json.dumps``.
-    """
     from gateway.config import PlatformConfig
     cfg = PlatformConfig(enabled=True, extra={"ws_url": "ws://localhost:5225"})
     adapter = SimplexAdapter(cfg)
@@ -246,11 +231,7 @@ async def test_send_group():
 
     result = await adapter.send("group:grp-99", "Hello, group!")
     payload = json.loads(mock_ws.send.call_args[0][0])
-    assert payload["cmd"].startswith("/_send #grp-99 json ")
-    msg_content = json.loads(payload["cmd"].split(" json ", 1)[1])[0][
-        "msgContent"
-    ]
-    assert msg_content == {"type": "text", "text": "Hello, group!"}
+    assert payload["cmd"] == "#[grp-99] Hello, group!"
     assert result.success is True