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/adapter-sx.sx

@@ -48,6 +48,9 @@
           (list)
           (aser-list expr env))
 
+      ;; Spread — pass through for client rendering
+      "spread"  expr
+
       :else expr)))