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/bootstrap_py.py

@@ -285,6 +285,14 @@ class PyEmitter:
             "svg-context-set!": "svg_context_set",
             "svg-context-reset!": "svg_context_reset",
             "css-class-collect!": "css_class_collect",
+            # spread + collect primitives
+            "make-spread": "make_spread",
+            "spread?": "is_spread",
+            "spread-attrs": "spread_attrs",
+            "merge-spread-attrs": "merge_spread_attrs",
+            "collect!": "sx_collect",
+            "collected": "sx_collected",
+            "clear-collected!": "sx_clear_collected",
             "is-raw-html?": "is_raw_html",
             "async-coroutine?": "is_async_coroutine",
             "async-await!": "async_await",