Data-first HO forms, fix plan pages, aser error handling (1080/1080)

Evaluator: data-first higher-order forms — ho-swap-args auto-detects
(map coll fn) vs (map fn coll), both work. Threading + HO: (-> data
(map fn)) dispatches through CEK HO machinery via quoted-value splice.
17 new tests in test-cek-advanced.sx.

Fix plan pages: add mother-language, isolated-evaluator, rust-wasm-host
to page-functions.sx plan() — were in defpage but missing from URL router.

Aser error handling: pages.py now catches EvalError separately, renders
visible error banner instead of silently sending empty content. All
except blocks include traceback in logs.

Scope primitives: register collect!/collected/clear-collected!/emitted/
emit!/context in shared/sx/primitives.py so hand-written _aser can
resolve them (fixes ~cssx/flush expansion failure).

New test file: shared/sx/tests/test_aser_errors.py — 19 pytest tests
for error propagation through all aser control flow forms.

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

shared/sx/primitives.py

@@ -573,3 +573,32 @@ def prim_json_encode(value) -> str:
     import json
     return json.dumps(value, indent=2)
 
+
+# ---------------------------------------------------------------------------
+# Scope primitives — delegate to sx_ref.py's scope stack implementation
+# (shared global state between transpiled and hand-written evaluators)
+# ---------------------------------------------------------------------------
+
+def _lazy_scope_primitives():
+    """Register scope/provide/collect primitives from sx_ref.py.
+
+    Called at import time — if sx_ref.py isn't built yet, silently skip.
+    These are needed by the hand-written _aser in async_eval.py when
+    expanding components that use scoped effects (e.g. ~cssx/flush).
+    """
+    try:
+        from .ref.sx_ref import (
+            sx_collect, sx_collected, sx_clear_collected,
+            sx_emitted, sx_emit, sx_context,
+        )
+        _PRIMITIVES["collect!"] = sx_collect
+        _PRIMITIVES["collected"] = sx_collected
+        _PRIMITIVES["clear-collected!"] = sx_clear_collected
+        _PRIMITIVES["emitted"] = sx_emitted
+        _PRIMITIVES["emit!"] = sx_emit
+        _PRIMITIVES["context"] = sx_context
+    except ImportError:
+        pass
+
+_lazy_scope_primitives()
+