Refactor spread to use provide/emit! internally

Spreads now emit their attrs into the nearest element's provide scope
instead of requiring per-child spread? checks at every intermediate
layer. emit! is tolerant (no-op when no provider), so spreads in
non-element contexts silently vanish.

- adapter-html: element/lake/marsh wrap children in provide, collect
  emitted; removed 14 spread filters from fragment, forms, components
- adapter-sx: aser wraps result to catch spread values from fn calls;
  aser-call uses provide with attr-parts/child-parts ordering
- adapter-async: same pattern for both render and aser paths
- adapter-dom: added emit! in spread dispatch + provide in element
  rendering; kept spread? checks for reactive/island and DOM safety
- platform: emit! returns NIL when no provider instead of erroring
- 3 new aser tests: stored spread, nested element, silent drop

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/test-aser.sx

@@ -285,6 +285,19 @@
     (assert-equal "(div :class \"card\" :style \"color:red\" \"hello\")"
       (render-sx "(div (make-spread {:class \"card\"}) (make-spread {:style \"color:red\"}) \"hello\")")))
 
-  (deftest "spread in fragment is filtered"
+  (deftest "spread in fragment is silently dropped"
     (assert-equal "(<> \"hello\")"
-      (render-sx "(<> (make-spread {:class \"card\"}) \"hello\")"))))
+      (render-sx "(<> (make-spread {:class \"card\"}) \"hello\")")))
+
+  (deftest "stored spread in let binding"
+    (assert-equal "(div :class \"card\" \"hello\")"
+      (render-sx "(let ((card (make-spread {:class \"card\"})))
+                    (div card \"hello\"))")))
+
+  (deftest "spread in nested element"
+    (assert-equal "(div (span :class \"inner\" \"hi\"))"
+      (render-sx "(div (span (make-spread {:class \"inner\"}) \"hi\"))")))
+
+  (deftest "spread in non-element context silently drops"
+    (assert-equal "hello"
+      (render-sx "(do (make-spread {:class \"card\"}) \"hello\")"))))