Nav redesign: embedded breadcrumb navigation with recursive depth

Replace menu bar navigation with in-page nav embedded in content area.
Each page shows logo/tagline/copyright, then a sibling row per trail
level (← prev | Current | next →), then children as button links.

- resolve-nav-path: recursive walk with no depth limit
- find-nav-index: rewritten with recursion (set! broken across closures)
- Walk stops on exact href match (prevents /cssx/ drilling into Overview)
- Unicode chars (©, ←, →) inline instead of \u escapes (SX parser doesn't support them)
- All 38 defpages wrapped in (~sx-doc :path ...) for in-page nav
- Layout returns only root header (nav moved out of blue menu bar)
- Standalone layout variants for sx-web.org (return nil)
- New plans: environment-images, runtime-slicing, typed-sx, nav-redesign, sx-web-platform

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

sx/sxc/home.sx

@@ -5,7 +5,7 @@
     (h1 :class "text-5xl font-bold text-stone-900 mb-4"
       (span :class "text-violet-600 font-mono" "(<sx>)"))
     (p :class "text-2xl text-stone-600 mb-4"
-      "s-expressions for the web")
+      "Framework free reactive hypermedia")
     (p :class "text-sm text-stone-400"
       "© Giles Bradshaw 2026")
     (p :class "text-lg text-stone-500 max-w-2xl mx-auto mb-12"

sx/sxc/pages/layouts.py

@@ -8,20 +8,12 @@ def _register_sx_layouts() -> None:
     from shared.sx.layouts import register_sx_layout
 
     if os.getenv("SX_STANDALONE") == "true":
-        register_sx_layout("sx",
-                           "sx-standalone-layout-full",
-                           "sx-standalone-layout-oob",
-                           "sx-standalone-layout-mobile")
-        register_sx_layout("sx-section",
-                           "sx-standalone-section-layout-full",
-                           "sx-standalone-section-layout-oob",
-                           "sx-standalone-section-layout-mobile")
+        register_sx_layout("sx-docs",
+                           "sx-standalone-docs-layout-full",
+                           "sx-standalone-docs-layout-oob",
+                           "sx-standalone-docs-layout-mobile")
     else:
-        register_sx_layout("sx",
-                           "sx-layout-full",
-                           "sx-layout-oob",
-                           "sx-layout-mobile")
-        register_sx_layout("sx-section",
-                           "sx-section-layout-full",
-                           "sx-section-layout-oob",
-                           "sx-section-layout-mobile")
+        register_sx_layout("sx-docs",
+                           "sx-docs-layout-full",
+                           "sx-docs-layout-oob",
+                           "sx-docs-layout-mobile")