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/boundary.sx

@@ -313,3 +313,61 @@
   :effects [mutation]
   :doc "Group multiple signal writes. Subscribers are notified once at the end,
         after all values have been updated.")
+
+
+;; --------------------------------------------------------------------------
+;; Tier 4: Spread + Collect — render-time attribute injection and accumulation
+;;
+;; `spread` is a new type: a dict of attributes that, when returned as a child
+;; of an HTML element, merges its attrs onto the parent element rather than
+;; rendering as content. This enables components like `~cssx/tw` to inject
+;; classes and styles onto their parent from inside the child list.
+;;
+;; `collect!` / `collected` are render-time accumulators. Values are collected
+;; into named buckets (with deduplication) during rendering and retrieved at
+;; flush points (e.g. a single <style> tag for all collected CSS rules).
+;; --------------------------------------------------------------------------
+
+(declare-tier :spread :source "render.sx")
+
+(declare-spread-primitive "make-spread"
+  :params (attrs)
+  :returns "spread"
+  :effects []
+  :doc "Create a spread value from an attrs dict. When this value appears as
+        a child of an HTML element, its attrs are merged onto the parent
+        element (class values joined, others overwritten).")
+
+(declare-spread-primitive "spread?"
+  :params (x)
+  :returns "boolean"
+  :effects []
+  :doc "Test whether a value is a spread.")
+
+(declare-spread-primitive "spread-attrs"
+  :params (s)
+  :returns "dict"
+  :effects []
+  :doc "Extract the attrs dict from a spread value.")
+
+(declare-spread-primitive "collect!"
+  :params (bucket value)
+  :returns "nil"
+  :effects [mutation]
+  :doc "Add value to a named render-time accumulator bucket. Values are
+        deduplicated (no duplicates added). Buckets persist for the duration
+        of the current render pass.")
+
+(declare-spread-primitive "collected"
+  :params (bucket)
+  :returns "list"
+  :effects []
+  :doc "Return all values collected in the named bucket during the current
+        render pass. Returns an empty list if the bucket doesn't exist.")
+
+(declare-spread-primitive "clear-collected!"
+  :params (bucket)
+  :returns "nil"
+  :effects [mutation]
+  :doc "Clear a named render-time accumulator bucket. Used at flush points
+        after emitting collected values (e.g. after writing a <style> tag).")