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_js.py

@@ -864,6 +864,11 @@ PREAMBLE = '''\
   function RawHTML(html) { this.html = html; }
   RawHTML.prototype._raw = true;
 
+  function SxSpread(attrs) { this.attrs = attrs || {}; }
+  SxSpread.prototype._spread = true;
+
+  var _collectBuckets = {};
+
   function isSym(x) { return x != null && x._sym === true; }
   function isKw(x) { return x != null && x._kw === true; }
 
@@ -1073,6 +1078,16 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
     return true;
   };
 ''',
+
+    "stdlib.spread": '''
+  // stdlib.spread — spread + collect primitives
+  PRIMITIVES["make-spread"] = makeSpread;
+  PRIMITIVES["spread?"] = isSpread;
+  PRIMITIVES["spread-attrs"] = spreadAttrs;
+  PRIMITIVES["collect!"] = sxCollect;
+  PRIMITIVES["collected"] = sxCollected;
+  PRIMITIVES["clear-collected!"] = sxClearCollected;
+''',
 }
 # Modules to include by default (all)
 _ALL_JS_MODULES = list(PRIMITIVES_JS_MODULES.keys())
@@ -1108,6 +1123,7 @@ PLATFORM_JS_PRE = '''
     if (x._component) return "component";
     if (x._island) return "island";
     if (x._signal) return "signal";
+    if (x._spread) return "spread";
     if (x._macro) return "macro";
     if (x._raw) return "raw-html";
     if (typeof Node !== "undefined" && x instanceof Node) return "dom-node";
@@ -1130,6 +1146,22 @@ PLATFORM_JS_PRE = '''
   }
   function makeThunk(expr, env) { return new Thunk(expr, env); }
 
+  function makeSpread(attrs) { return new SxSpread(attrs || {}); }
+  function isSpread(x) { return x != null && x._spread === true; }
+  function spreadAttrs(s) { return s && s._spread ? s.attrs : {}; }
+
+  function sxCollect(bucket, value) {
+    if (!_collectBuckets[bucket]) _collectBuckets[bucket] = [];
+    var items = _collectBuckets[bucket];
+    if (items.indexOf(value) === -1) items.push(value);
+  }
+  function sxCollected(bucket) {
+    return _collectBuckets[bucket] ? _collectBuckets[bucket].slice() : [];
+  }
+  function sxClearCollected(bucket) {
+    if (_collectBuckets[bucket]) _collectBuckets[bucket] = [];
+  }
+
   function lambdaParams(f) { return f.params; }
   function lambdaBody(f) { return f.body; }
   function lambdaClosure(f) { return f.closure; }
@@ -3154,6 +3186,12 @@ def public_api_js(has_html, has_sx, has_dom, has_engine, has_orch, has_boot, has
         api_lines.append('    emitEvent: emitEvent,')
         api_lines.append('    onEvent: onEvent,')
         api_lines.append('    bridgeEvent: bridgeEvent,')
+    api_lines.append('    makeSpread: makeSpread,')
+    api_lines.append('    isSpread: isSpread,')
+    api_lines.append('    spreadAttrs: spreadAttrs,')
+    api_lines.append('    collect: sxCollect,')
+    api_lines.append('    collected: sxCollected,')
+    api_lines.append('    clearCollected: sxClearCollected,')
     api_lines.append(f'    _version: "{version}"')
     api_lines.append('  };')
     api_lines.append('')