Phase 2: IO detection & selective expansion in deps.sx

Extend the spec with IO scanning functions (scan-io-refs, transitive-io-refs,
compute-all-io-refs, component-pure?) that detect IO primitive references in
component ASTs. Components are classified as pure (no IO deps, safe for client
rendering) or IO-dependent (must expand server-side).

The partial evaluator (_aser) now uses per-component IO metadata instead of
the global _expand_components toggle: IO-dependent components expand server-
side, pure components serialize for client. Layout slot context still expands
all components for backwards compat.

Spec: 5 new functions + 2 platform interface additions in deps.sx
Host: io_refs field + is_pure property on Component, compute_all_io_refs()
Bootstrap: both sx_ref.py and sx-ref.js updated with IO functions
Bundle analyzer: shows pure/IO classification per page

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

sx/sxc/pages/docs.sx

@@ -414,7 +414,8 @@
     :selected (or (find-current isomorphism-nav-items slug) ""))
   :content (case slug
     "bundle-analyzer" (~bundle-analyzer-content
-      :pages pages :total-components total-components :total-macros total-macros)
+      :pages pages :total-components total-components :total-macros total-macros
+      :pure-count pure-count :io-count io-count)
     :else (~plan-isomorphic-content)))
 
 (defpage bundle-analyzer
@@ -428,7 +429,8 @@
     :selected "Bundle Analyzer")
   :data (bundle-analyzer-data)
   :content (~bundle-analyzer-content
-    :pages pages :total-components total-components :total-macros total-macros))
+    :pages pages :total-components total-components :total-macros total-macros
+    :pure-count pure-count :io-count io-count))
 
 ;; ---------------------------------------------------------------------------
 ;; Plans section

sx/sxc/pages/helpers.py

@@ -277,6 +277,8 @@ def _bundle_analyzer_data() -> dict:
     env = get_component_env()
     total_components = sum(1 for v in env.values() if isinstance(v, Component))
     total_macros = sum(1 for v in env.values() if isinstance(v, Macro))
+    pure_count = sum(1 for v in env.values() if isinstance(v, Component) and v.is_pure)
+    io_count = total_components - pure_count
 
     pages_data = []
     for name, page_def in sorted(get_all_pages("sx").items()):
@@ -286,6 +288,20 @@ def _bundle_analyzer_data() -> dict:
         n = len(needed)
         pct = round(n / total_components * 100) if total_components else 0
         savings = 100 - pct
+
+        # IO classification for components in this page
+        pure_in_page = 0
+        io_in_page = 0
+        page_io_refs: set[str] = set()
+        for comp_name in needed:
+            val = env.get(comp_name)
+            if isinstance(val, Component):
+                if val.is_pure:
+                    pure_in_page += 1
+                else:
+                    io_in_page += 1
+                    page_io_refs.update(val.io_refs)
+
         pages_data.append({
             "name": name,
             "path": page_def.path,
@@ -293,6 +309,9 @@ def _bundle_analyzer_data() -> dict:
             "needed": n,
             "pct": pct,
             "savings": savings,
+            "io-refs": len(page_io_refs),
+            "pure-in-page": pure_in_page,
+            "io-in-page": io_in_page,
         })
 
     pages_data.sort(key=lambda p: p["needed"], reverse=True)
@@ -301,6 +320,8 @@ def _bundle_analyzer_data() -> dict:
         "pages": pages_data,
         "total-components": total_components,
         "total-macros": total_macros,
+        "pure-count": pure_count,
+        "io-count": io_count,
     }