aser_slot migration: single-pass expansion, pipe desync fix, _render_to_sx

Three fixes completing the aser_slot migration:

1. Single-pass full-page rendering: eval_sx_url builds layout+content
   AST and aser_slots it in ONE call — avoids double-aser where
   re-parsed content hits "Undefined symbol: title/deref" errors.

2. Pipe desync fix: _inject_helpers_locked runs INSIDE the aser_slot
   lock acquisition (not as a separate lock). Prevents interleaved
   commands from other coroutines between injection and aser-slot.

3. _render_to_sx uses aser_slot (not aser): layout wrappers like
   oob_page_sx contain re-parsed content from earlier aser_slot
   calls. Regular aser fails on symbols that were bound during
   the earlier expansion. aser_slot handles them correctly.

HTMX path: aser_slot the content, then oob_page_sx wraps it.
Full page path: build (~shared:layout/app-body :content wrapped_ast),
aser_slot in one pass, pass directly to sx_page.

New Playwright tests: test_navigate_geography_to_reactive,
test_direct_load_reactive_page.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/tests/test_demos.py

@@ -537,6 +537,32 @@ class TestHomePage:
         assert not fatal, f"JS errors after navigation: {fatal}"
 
 
+    def test_navigate_geography_to_reactive(self, page: Page):
+        """Click Reactive Islands from Geography — content must render."""
+        errors = []
+        page.on("pageerror", lambda err: errors.append(f"UNCAUGHT: {err.message}"))
+        nav(page, "(geography)")
+        ri_link = page.locator("a[sx-push-url]:has-text('Reactive Islands')").first
+        expect(ri_link).to_be_visible(timeout=10000)
+        ri_link.click()
+        page.wait_for_timeout(5000)
+        expect(page.locator("#main-panel")).to_contain_text("Reactive Islands", timeout=10000)
+        fatal = [e for e in errors if "Not callable" in e or "Undefined symbol" in e or "UNCAUGHT" in e]
+        assert not fatal, f"JS errors after navigation: {fatal}"
+
+    def test_direct_load_reactive_page(self, page: Page):
+        """Direct load of reactive islands page — no errors, content renders."""
+        errors = []
+        page.on("pageerror", lambda err: errors.append(f"UNCAUGHT: {err.message}"))
+        page.on("console", lambda msg: errors.append(msg.text) if msg.type == "error" else None)
+        page.goto(f"{BASE}/sx/(geography.(reactive))", wait_until="networkidle")
+        # aser_slot may take several seconds on first load — wait for render
+        page.wait_for_selector("#main-panel", timeout=30000)
+        expect(page.locator("#main-panel")).to_contain_text("Reactive Islands", timeout=10000)
+        fatal = [e for e in errors if "Not callable" in e or "Undefined symbol" in e or "SES_UNCAUGHT" in e or "UNCAUGHT" in e]
+        assert not fatal, f"JS errors on reactive page: {fatal}"
+
+
 class TestDocPages:
     @pytest.mark.parametrize("path,expected", [
         ("(geography.(reactive))", "Reactive Islands"),