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>

shared/sx/ref/bootstrap_py.py

@@ -247,9 +247,17 @@ class PyEmitter:
             "component-deps": "component_deps",
             "component-set-deps!": "component_set_deps",
             "component-css-classes": "component_css_classes",
+            "component-io-refs": "component_io_refs",
+            "component-set-io-refs!": "component_set_io_refs",
             "env-components": "env_components",
             "regex-find-all": "regex_find_all",
             "scan-css-classes": "scan_css_classes",
+            # deps.sx IO detection
+            "scan-io-refs": "scan_io_refs",
+            "scan-io-refs-walk": "scan_io_refs_walk",
+            "transitive-io-refs": "transitive_io_refs",
+            "compute-all-io-refs": "compute_all_io_refs",
+            "component-pure?": "component_pure_p",
         }
         if name in RENAMES:
             return RENAMES[name]
@@ -1982,6 +1990,14 @@ PLATFORM_DEPS_PY = (
     '    for m in _re.finditer(r\';;\\s*@css\\s+(.+)\', source):\n'
     '        classes.update(m.group(1).split())\n'
     '    return list(classes)\n'
+    '\n'
+    'def component_io_refs(c):\n'
+    '    """Return cached IO refs list for a component (may be empty)."""\n'
+    '    return list(c.io_refs) if hasattr(c, "io_refs") and c.io_refs else []\n'
+    '\n'
+    'def component_set_io_refs(c, refs):\n'
+    '    """Cache IO refs on a component."""\n'
+    '    c.io_refs = set(refs) if not isinstance(refs, set) else refs\n'
 )