Add SVG namespace auto-detection, custom elements, html: prefix, and fix filter/map tag collision

- Fix filter/map dispatching as HO functions when used as SVG/HTML tags
  (peek at first arg — Keyword means tag call, not function call)
- Add html: prefix escape hatch to force any name to render as an element
- Support custom elements (hyphenated names) per Web Components spec
- SVG/MathML namespace auto-detection: client threads ns param through
  render chain; server uses _svg_context ContextVar so unknown tags
  inside (svg ...) or (math ...) render as elements without enumeration

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

shared/sx/html.py

@@ -44,6 +44,12 @@ css_class_collector: contextvars.ContextVar[set[str] | None] = contextvars.Conte
     "css_class_collector", default=None
 )
 
+# ContextVar for SVG/MathML namespace auto-detection.
+# When True, unknown tag names inside (svg ...) or (math ...) are treated as elements.
+_svg_context: contextvars.ContextVar[bool] = contextvars.ContextVar(
+    "_svg_context", default=False
+)
+
 
 class _RawHTML:
     """Marker for pre-rendered HTML that should not be escaped."""
@@ -430,10 +436,19 @@ def _render_list(expr: list, env: dict[str, Any]) -> str:
         if name == "<>":
             return "".join(_render(child, env) for child in expr[1:])
 
+        # --- html: prefix → force tag rendering --------------------------
+        if name.startswith("html:"):
+            return _render_element(name[5:], expr[1:], env)
+
         # --- Render-aware special forms --------------------------------------
         # Check BEFORE HTML_TAGS because some names overlap (e.g. `map`).
-        if name in _RENDER_FORMS:
-            return _RENDER_FORMS[name](expr, env)
+        # 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.
+        rsf = _RENDER_FORMS.get(name)
+        if rsf is not None:
+            if name in HTML_TAGS and len(expr) > 1 and isinstance(expr[1], Keyword):
+                return _render_element(name, expr[1:], env)
+            return rsf(expr, env)
 
         # --- Macro expansion → expand then render --------------------------
         if name in env:
@@ -453,6 +468,14 @@ def _render_list(expr: list, env: dict[str, Any]) -> str:
                 return _render_component(val, expr[1:], env)
             # Fall through to evaluation
 
+        # --- Custom element (hyphenated name) → render as tag ------------
+        if "-" in name:
+            return _render_element(name, expr[1:], env)
+
+        # --- SVG/MathML context → unknown names are child elements --------
+        if _svg_context.get(False):
+            return _render_element(name, expr[1:], env)
+
         # --- Other special forms / function calls → evaluate then render ---
         result = _eval(expr, env)
         return _render(result, env)
@@ -524,7 +547,15 @@ def _render_element(tag: str, args: list, env: dict[str, Any]) -> str:
     if tag in VOID_ELEMENTS:
         return opening
 
-    # Render children
-    child_html = "".join(_render(child, env) for child in children)
+    # SVG/MathML namespace auto-detection: set context for children
+    token = None
+    if tag in ("svg", "math"):
+        token = _svg_context.set(True)
+
+    try:
+        child_html = "".join(_render(child, env) for child in children)
+    finally:
+        if token is not None:
+            _svg_context.reset(token)
 
     return f"{opening}{child_html}</{tag}>"