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