Fix island hydration double-render bug, add marshes plan

Client-rendered islands were re-hydrated by boot.sx because
renderDomIsland didn't mark them as processed. Hydration read
empty data-sx-state, overwriting kwargs (e.g. path) with NIL.
Fix: mark-processed! in adapter-dom.sx so boot skips them.

New plan: marshes — where reactivity and hypermedia interpenetrate.
Three patterns: server writes to signals, reactive marsh zones with
transforms, and signal-bound hypermedia interpretation.

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

sx/sxc/pages/docs.sx

@@ -541,6 +541,7 @@
       "event-bridge" (~reactive-islands-event-bridge-content)
       "named-stores" (~reactive-islands-named-stores-content)
       "plan" (~reactive-islands-plan-content)
+      "marshes" (~reactive-islands-marshes-content)
       "phase2" (~reactive-islands-phase2-content)
       :else (~reactive-islands-index-content))))