Fix pipe desync: async drain on _send, robust Playwright tests

Root cause: OcamlBridge._send() used write() without drain().
asyncio.StreamWriter buffers writes — without drain(), multiple
commands accumulate and flush as a batch. The kernel processes
them sequentially, sending responses, but Python only reads one
response per command → pipe desync → "unexpected response" errors.

Fix: _send() is now async, calls drain() after every write.
All 14 callers updated to await.

Playwright tests rewritten:
- test_home_has_header: verifies #logo-opacity visible (was only
  checking for "sx" text — never caught missing header)
- test_home_has_nav_children: Geography link must be visible
- test_home_has_main_panel: #main-panel must have child elements
- TestDirectPageLoad: fresh browser.new_context() per test to
  avoid stale component hash in localStorage
- _setup_error_capture + _check_no_fatal_errors helpers

_render_to_sx uses aser_slot (not aser) — layout wrappers contain
re-parsed content that needs full expansion capability.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sxc/pages/sx_router.py

@@ -216,18 +216,8 @@ async def eval_sx_url(raw_path: str) -> Any:
                     Keyword("content"), wrapped_ast,
                 ]
                 full_text = serialize(full_ast)
-                has_nl = chr(10) in full_text
-                if has_nl:
-                    logger.error("NEWLINE in aser_slot input at char %d!",
-                                 full_text.index(chr(10)))
-                import time as _time
-                _t0 = _time.monotonic()
                 body_sx = SxExpr(await bridge.aser_slot(
                     full_text, ctx=ocaml_ctx))
-                _elapsed = _time.monotonic() - _t0
-                logger.info("aser_slot: %.1fs, input=%d chars, output=%d chars, starts=%s",
-                            _elapsed, len(full_text), len(body_sx),
-                            str(body_sx)[:100])
                 tctx = await get_template_context()
                 return await make_response(
                     await sx_page(tctx, body_sx), 200)