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/sxc/pages/sx_router.py

@@ -90,7 +90,7 @@ async def eval_sx_url(raw_path: str) -> Any:
     from shared.sx.jinja_bridge import get_component_env, _get_request_context
     from shared.sx.pages import get_page, get_page_helpers, _eval_slot
     from shared.sx.types import Symbol, Keyword
-    from shared.sx.helpers import full_page_sx, oob_page_sx, sx_response
+    from shared.sx.helpers import full_page_sx, oob_page_sx, sx_response, sx_page
     from shared.sx.page import get_template_context
     from shared.browser.app.utils.htmx import is_htmx_request
     from shared.sx.ref.sx_ref import prepare_url_expr
@@ -200,16 +200,44 @@ async def eval_sx_url(raw_path: str) -> Any:
             from shared.sx.parser import serialize
             from shared.sx.types import SxExpr
             bridge = await get_bridge()
-            sx_text = serialize(wrapped_ast)
             ocaml_ctx = {"_helper_service": "sx"}
-            content_sx = SxExpr(await bridge.aser(sx_text, ctx=ocaml_ctx))
+
+            if is_htmx_request():
+                # HTMX: aser_slot the content, wrap in OOB layout
+                content_sx = SxExpr(await bridge.aser_slot(
+                    serialize(wrapped_ast), ctx=ocaml_ctx))
+                return sx_response(await oob_page_sx(content=content_sx))
+            else:
+                # Full page: build layout+content AST and aser_slot
+                # in ONE pass — avoids double-aser that breaks when
+                # re-parsed content contains islands/reactive symbols.
+                full_ast = [
+                    Symbol("~shared:layout/app-body"),
+                    Keyword("content"), wrapped_ast,
+                ]
+                full_text = serialize(full_ast)
+                has_nl = chr(10) in full_text
+                if has_nl:
+                    logger.error("NEWLINE in aser_slot input at char %d!",
+                                 full_text.index(chr(10)))
+                import time as _time
+                _t0 = _time.monotonic()
+                body_sx = SxExpr(await bridge.aser_slot(
+                    full_text, ctx=ocaml_ctx))
+                _elapsed = _time.monotonic() - _t0
+                logger.info("aser_slot: %.1fs, input=%d chars, output=%d chars, starts=%s",
+                            _elapsed, len(full_text), len(body_sx),
+                            str(body_sx)[:100])
+                tctx = await get_template_context()
+                return await make_response(
+                    await sx_page(tctx, body_sx), 200)
         else:
             content_sx = await _eval_slot(wrapped_ast, env, ctx)
     except Exception as e:
         logger.error("SX URL render failed for %s: %s", raw_path, e, exc_info=True)
         return None
 
-    # Return response — Python wraps in page shell (CSS, scripts, headers)
+    # Return response (Python path)
     if is_htmx_request():
         return sx_response(await oob_page_sx(content=content_sx))
     else: