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/render.sx

@@ -213,6 +213,43 @@
                      (= (type-of (nth expr 1)) "keyword")))))))))
 
 
+;; --------------------------------------------------------------------------
+;; Spread — attribute injection from children into parent elements
+;; --------------------------------------------------------------------------
+;;
+;; A spread value is a dict of attributes that, when returned as a child
+;; of an HTML element, merges its attrs onto the parent element.
+;; This enables components to inject classes/styles/data-attrs onto their
+;; parent without the parent knowing about the specific attrs.
+;;
+;; merge-spread-attrs: merge a spread's attrs into an element's attrs dict.
+;; Class values are joined (space-separated); others overwrite.
+;; Mutates the target attrs dict in place.
+
+(define merge-spread-attrs :effects [mutation]
+  (fn ((target :as dict) (spread-dict :as dict))
+    (for-each
+      (fn ((key :as string))
+        (let ((val (dict-get spread-dict key)))
+          (if (= key "class")
+            ;; Class: join existing + new with space
+            (let ((existing (dict-get target "class")))
+              (dict-set! target "class"
+                (if (and existing (not (= existing "")))
+                  (str existing " " val)
+                  val)))
+            ;; Style: join with semicolons
+            (if (= key "style")
+              (let ((existing (dict-get target "style")))
+                (dict-set! target "style"
+                  (if (and existing (not (= existing "")))
+                    (str existing ";" val)
+                    val)))
+              ;; Everything else: overwrite
+              (dict-set! target key val)))))
+      (keys spread-dict))))
+
+
 ;; --------------------------------------------------------------------------
 ;; Platform interface (shared across adapters)
 ;; --------------------------------------------------------------------------
@@ -222,6 +259,16 @@
 ;;   (escape-attr s)           → attribute-value-escaped string
 ;;   (raw-html-content r)      → unwrap RawHTML marker to string
 ;;
+;; Spread (render-time attribute injection):
+;;   (make-spread attrs)       → Spread value
+;;   (spread? x)               → boolean
+;;   (spread-attrs s)          → dict
+;;
+;; Render-time accumulators:
+;;   (collect! bucket value)   → void
+;;   (collected bucket)        → list
+;;   (clear-collected! bucket) → void
+;;
 ;; From parser.sx:
 ;;   (sx-serialize val)        → SX source string (aliased as serialize above)
 ;; --------------------------------------------------------------------------