Add predictive component prefetching plan to sx-docs

4-phase design: server endpoint for on-demand component defs,
SX-specced client prefetch logic (hover/viewport triggers),
boundary declarations, and bootstrap integration.

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

sx/sxc/pages/docs.sx

@@ -477,4 +477,5 @@
   :content (case slug
     "reader-macros" (~plan-reader-macros-content)
     "sx-activity" (~plan-sx-activity-content)
+    "predictive-prefetch" (~plan-predictive-prefetch-content)
     :else (~plans-index-content)))