Stream extra component defs with resolve scripts

Resolved SX content may reference components not in the initial shell
scan (e.g. ~cart-mini from IO-generated headers). Diff the needed
components against what the shell already sent and prepend any extra
defcomps as a <script type="text/sx"> block before the resolve script.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/helpers.py

@@ -944,13 +944,22 @@ def sx_page_streaming_parts(ctx: dict, page_html: str, *,
     return shell, tail
 
 
-def sx_streaming_resolve_script(suspension_id: str, sx_source: str) -> str:
-    """Build a <script> tag that resolves a streaming suspense placeholder."""
+def sx_streaming_resolve_script(suspension_id: str, sx_source: str,
+                                extra_components: str = "") -> str:
+    """Build a <script> tag that resolves a streaming suspense placeholder.
+
+    If *extra_components* is non-empty, a ``<script type="text/sx">`` block
+    is prepended so the client loads those component defs before resolving.
+    """
     import json
-    return _SX_STREAMING_RESOLVE.format(
+    parts = []
+    if extra_components:
+        parts.append(f'<script type="text/sx">{extra_components}</script>')
+    parts.append(_SX_STREAMING_RESOLVE.format(
         id=json.dumps(suspension_id),
         sx=json.dumps(sx_source),
-    )
+    ))
+    return "\n".join(parts)
 
 
 _SCRIPT_HASH_CACHE: dict[str, str] = {}