Add Spreads page under Geography — spread/collect/reactive-spread docs

Documents the three orthogonal primitives (spread, collect!, reactive-spread),
their operation across server/client/morph boundaries, CSSX as use case,
semantic style variables, and the planned provide/context/emit! unification.

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

sx/sxc/pages/docs.sx

@@ -611,6 +611,16 @@
       "phase2" (~reactive-islands/phase2/reactive-islands-phase2-content)
       :else (~reactive-islands/index/reactive-islands-index-content))))
 
+;; ---------------------------------------------------------------------------
+;; Spreads section (under Geography)
+;; ---------------------------------------------------------------------------
+
+(defpage spreads-index
+  :path "/geography/spreads/"
+  :auth :public
+  :layout :sx-docs
+  :content (~layouts/doc :path "/sx/(geography.(spreads))" (~geography/spreads-content)))
+
 ;; ---------------------------------------------------------------------------
 ;; Marshes section (under Geography)
 ;; ---------------------------------------------------------------------------

sx/sxc/pages/sx_router.py

@@ -267,6 +267,8 @@ _REDIRECT_PATTERNS = [
      lambda m: f"/sx/(geography.(reactive.{m.group(1)}))"),
     (re.compile(r"^/geography/isomorphism/(.+?)/?$"),
      lambda m: f"/sx/(geography.(isomorphism.{m.group(1)}))"),
+    (re.compile(r"^/geography/spreads/?$"),
+     "/sx/(geography.(spreads))"),
     (re.compile(r"^/geography/marshes/?$"),
      "/sx/(geography.(marshes))"),
     (re.compile(r"^/applications/cssx/(.+?)/?$"),