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/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/?$"),
      "/sx/(geography.(marshes))"),
     (re.compile(r"^/applications/cssx/(.+?)/?$"),