Add hub iframe embed shell with tab fragment API.

Replace full-page soft nav with a persistent shell and /api/embed/page loads so tab switches in the hub iframe avoid document.write flicker.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_instance_embed_lib.py

@@ -0,0 +1,26 @@
+from instance_embed_lib import (
+    EMBED_TABS,
+    path_to_embed_tab,
+    rewrite_embed_dest,
+)
+
+
+def test_path_to_embed_tab():
+    assert path_to_embed_tab("/trade") == "trade"
+    assert path_to_embed_tab("/key_monitor") == "key_monitor"
+    assert path_to_embed_tab("/strategy/records") == "strategy_records"
+    assert path_to_embed_tab("/unknown") is None
+
+
+def test_rewrite_embed_dest():
+    url = rewrite_embed_dest("/trade", hub_theme="dark")
+    assert url.startswith("/embed?")
+    assert "tab=trade" in url
+    assert "embed=1" in url
+    assert "hub_theme=dark" in url
+
+
+def test_embed_tabs_cover_main_nav():
+    assert "trade" in EMBED_TABS
+    assert "key_monitor" in EMBED_TABS
+    assert "records" in EMBED_TABS