Add spread + collect primitives, rewrite ~cssx/tw as defcomp

New SX primitives for child-to-parent communication in the render tree:
- spread type: make-spread, spread?, spread-attrs — child injects attrs
  onto parent element (class joins with space, style with semicolon)
- collect!/collected/clear-collected! — render-time accumulation with
  dedup into named buckets

~cssx/tw is now a proper defcomp returning a spread value instead of a
macro wrapping children. ~cssx/flush reads collected "cssx" rules and
emits a single <style data-cssx> tag.

All four render adapters (html, async, dom, aser) handle spread values.
Both bootstraps (Python + JS) regenerated. Also fixes length→len in
cssx.sx (length was never a registered primitive).

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

shared/sx/ref/platform_py.py

@@ -84,6 +84,23 @@ class _RawHTML:
         self.html = html
 
 
+class _Spread:
+    """Attribute injection value — merges attrs onto parent element."""
+    __slots__ = ("attrs",)
+    def __init__(self, attrs: dict):
+        self.attrs = dict(attrs) if attrs else {}
+
+
+# Render-time accumulator buckets (per render pass)
+_collect_buckets: dict[str, list] = {}
+
+
+def _collect_reset():
+    """Reset all collect buckets (call at start of each render pass)."""
+    global _collect_buckets
+    _collect_buckets = {}
+
+
 def sx_truthy(x):
     """SX truthiness: everything is truthy except False, None, and NIL."""
     if x is False:
@@ -167,6 +184,8 @@ def type_of(x):
         return "island"
     if isinstance(x, _Signal):
         return "signal"
+    if isinstance(x, _Spread):
+        return "spread"
     if isinstance(x, Macro):
         return "macro"
     if isinstance(x, _RawHTML):
@@ -270,6 +289,38 @@ def make_thunk(expr, env):
     return _Thunk(expr, env)
 
 
+def make_spread(attrs):
+    return _Spread(attrs if isinstance(attrs, dict) else {})
+
+
+def is_spread(x):
+    return isinstance(x, _Spread)
+
+
+def spread_attrs(s):
+    return s.attrs if isinstance(s, _Spread) else {}
+
+
+def sx_collect(bucket, value):
+    """Add value to named render-time accumulator (deduplicated)."""
+    if bucket not in _collect_buckets:
+        _collect_buckets[bucket] = []
+    items = _collect_buckets[bucket]
+    if value not in items:
+        items.append(value)
+
+
+def sx_collected(bucket):
+    """Return all values in named render-time accumulator."""
+    return list(_collect_buckets.get(bucket, []))
+
+
+def sx_clear_collected(bucket):
+    """Clear a named render-time accumulator bucket."""
+    if bucket in _collect_buckets:
+        _collect_buckets[bucket] = []
+
+
 def lambda_params(f):
     return f.params
 
@@ -881,6 +932,16 @@ def _strip_tags(s):
     "stdlib.debug": '''
 # stdlib.debug
 PRIMITIVES["assert"] = lambda cond, msg="Assertion failed": (_ for _ in ()).throw(RuntimeError(f"Assertion error: {msg}")) if not sx_truthy(cond) else True
+''',
+
+    "stdlib.spread": '''
+# stdlib.spread — spread + collect primitives
+PRIMITIVES["make-spread"] = make_spread
+PRIMITIVES["spread?"] = is_spread
+PRIMITIVES["spread-attrs"] = spread_attrs
+PRIMITIVES["collect!"] = sx_collect
+PRIMITIVES["collected"] = sx_collected
+PRIMITIVES["clear-collected!"] = sx_clear_collected
 ''',
 }