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/agent/test_memory_provider.py

@@ -229,7 +229,6 @@ class TestMemoryManager:
         mgr.add_provider(p2)
 
         mgr.queue_prefetch_all("next turn")
-        mgr.flush_pending(timeout=5)
         assert p1.queued_prefetches == ["next turn"]
         assert p2.queued_prefetches == ["next turn"]
 
@@ -241,7 +240,6 @@ class TestMemoryManager:
         mgr.add_provider(p2)
 
         mgr.sync_all("user msg", "assistant msg")
-        mgr.flush_pending(timeout=5)
         assert p1.synced_turns == [("user msg", "assistant msg")]
         assert p2.synced_turns == [("user msg", "assistant msg")]
 
@@ -255,7 +253,7 @@ class TestMemoryManager:
         ]
 
         mgr.sync_all("user msg", "assistant msg", session_id="sess-1", messages=messages)
-        mgr.flush_pending(timeout=5)
+
         assert p.synced_turns == [("user msg", "assistant msg", "sess-1", messages)]
 
     def test_sync_all_omits_messages_for_legacy_provider(self):
@@ -264,7 +262,7 @@ class TestMemoryManager:
         mgr.add_provider(p)
 
         mgr.sync_all("user msg", "assistant msg", messages=[{"role": "tool"}])
-        mgr.flush_pending(timeout=5)
+
         assert p.synced_turns == [("user msg", "assistant msg")]
 
     def test_sync_failure_doesnt_block_others(self):
@@ -277,7 +275,6 @@ class TestMemoryManager:
         mgr.add_provider(p2)
 
         mgr.sync_all("user", "assistant")
-        mgr.flush_pending(timeout=5)
         # p1 failed but p2 still synced
         assert p2.synced_turns == [("user", "assistant")]