Fix island dep scanning + spread-through-reactive-if debug

deps.sx: scan island bodies for component deps (was only scanning
"component" and "macro", missing "island" type). This ensures
~cssx/tw and its dependencies are sent to the client for islands.

cssx.sx: move if inside make-spread arg so it's evaluated by
eval-expr (no reactive wrapping) instead of render-to-dom which
applies reactive-if inside island scope, converting the spread
into a fragment and losing the class attrs.

Added island dep tests at 3 levels: test-deps.sx (spec),
test_deps.py (Python), test_parity.py (ref vs fallback).

sx-browser.js: temporary debug logging at spread detection points.

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

shared/sx/templates/cssx.sx

@@ -476,10 +476,12 @@
         (classes (map (fn (r) (get r "cls")) valid))
         (rules (map (fn (r) (get r "rule")) valid))
         (_ (for-each (fn (rule) (collect! "cssx" rule)) rules)))
-    ;; Return spread: injects class + data-tw onto parent element
-    (if (empty? classes)
-      nil
-      (make-spread {"class" (join " " classes)
+    ;; Return spread: injects class + data-tw onto parent element.
+    ;; The if is inside make-spread's arg so it goes through eval-expr
+    ;; (not render-to-dom), avoiding reactive-if wrapping in islands.
+    (make-spread (if (empty? classes)
+                   {}
+                   {"class" (join " " classes)
                     "data-tw" (or tokens "")}))))