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/tests/test_deps.py

@@ -113,6 +113,17 @@ class TestTransitiveDeps:
         deps = transitive_deps("~card", env)
         assert "~unknown" in deps
 
+    def test_island_deps_scanned(self):
+        """Island bodies must be scanned for component dependencies."""
+        env = make_env(
+            '(defcomp ~leaf (&key) (span "leaf"))',
+            '(defcomp ~branch (&key) (div (~leaf)))',
+            '(defisland ~my-island () (div (~branch) "island"))',
+        )
+        deps = transitive_deps("~my-island", env)
+        assert "~branch" in deps
+        assert "~leaf" in deps
+
     def test_without_tilde_prefix(self):
         env = make_env(
             '(defcomp ~card (&key) (div (~shared:misc/badge)))',