Fix filter/map tag disambiguation inside SVG context without keyword attrs

(filter (feTurbulence ...)) inside (svg ...) has no keyword first arg,
so the keyword-only check dispatched it as a HO function. Now also
check SVG/MathML context (ns in client, _svg_context in server).

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

shared/sx/html.py

@@ -442,11 +442,14 @@ def _render_list(expr: list, env: dict[str, Any]) -> str:
 
         # --- Render-aware special forms --------------------------------------
         # Check BEFORE HTML_TAGS because some names overlap (e.g. `map`).
-        # But if the name is ALSO an HTML tag and first arg is a Keyword,
-        # it's a tag call (e.g. (filter :id "x" ...)), not a HO function.
+        # But if the name is ALSO an HTML tag and (a) first arg is a Keyword
+        # or (b) we're inside SVG/MathML context, it's a tag call.
         rsf = _RENDER_FORMS.get(name)
         if rsf is not None:
-            if name in HTML_TAGS and len(expr) > 1 and isinstance(expr[1], Keyword):
+            if name in HTML_TAGS and (
+                (len(expr) > 1 and isinstance(expr[1], Keyword))
+                or _svg_context.get(False)
+            ):
                 return _render_element(name, expr[1:], env)
             return rsf(expr, env)