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-dom.sx

@@ -44,6 +44,9 @@
       ;; Pre-rendered DOM node → pass through
       "dom-node"  expr
 
+      ;; Spread → pass through (parent element handles it)
+      "spread"    expr
+
       ;; Dict → empty
       "dict"      (create-fragment)
 
@@ -221,10 +224,34 @@
                             (dom-set-attr el attr-name (str attr-val)))))
                   (assoc state "skip" true "i" (inc (get state "i"))))
 
-                ;; Positional arg → child
+                ;; Positional arg → child (or spread → merge attrs onto element)
                 (do
                   (when (not (contains? VOID_ELEMENTS tag))
-                    (dom-append el (render-to-dom arg env new-ns)))
+                    (let ((child (render-to-dom arg env new-ns)))
+                      (if (spread? child)
+                        ;; Spread: merge attrs onto parent element
+                        (for-each
+                          (fn ((key :as string))
+                            (let ((val (dict-get (spread-attrs child) key)))
+                              (if (= key "class")
+                                ;; Class: append to existing
+                                (let ((existing (dom-get-attr el "class")))
+                                  (dom-set-attr el "class"
+                                    (if (and existing (not (= existing "")))
+                                      (str existing " " val)
+                                      val)))
+                                (if (= key "style")
+                                  ;; Style: append with semicolon
+                                  (let ((existing (dom-get-attr el "style")))
+                                    (dom-set-attr el "style"
+                                      (if (and existing (not (= existing "")))
+                                        (str existing ";" val)
+                                        val)))
+                                  ;; Other attrs: overwrite
+                                  (dom-set-attr el key (str val))))))
+                          (keys (spread-attrs child)))
+                        ;; Normal child: append to element
+                        (dom-append el child))))
                   (assoc state "i" (inc (get state "i"))))))))
         (dict "i" 0 "skip" false)
         args)