GraphSX URL routing: s-expression URLs for sx-docs

Replace path-based URLs with nested s-expression URLs across the sx app.
URLs like /language/docs/introduction become /(language.(doc.introduction)),
making the URL simultaneously a query, render instruction, and address.

- Add sx_router.py: catch-all route evaluator with dot→space conversion,
  auto-quoting slugs, two-phase eval, streaming detection, 301 redirects
- Add page-functions.sx: section + page functions for URL dispatch
- Rewrite nav-data.sx: ~200 hrefs to SX expression format, tree-descent
  nav matching via has-descendant-href? (replaces prefix heuristics)
- Convert ~120 old-style hrefs across 26 .sx content files
- Add SX Protocol proposal (etc/plans/sx-protocol)
- Wire catch-all route in app.py with before_request redirect handler

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

sx/app.py

@@ -114,10 +114,37 @@ def create_app() -> "Quart":
     from shared.sx.pages import auto_mount_pages
     auto_mount_pages(app, "sx")
 
+    # --- GraphSX catch-all route: SX expression URLs ---
+    from sxc.pages.sx_router import eval_sx_url, redirect_old_url
+
+    @app.before_request
+    async def sx_url_redirect():
+        """Redirect old-style paths to SX expression URLs (301)."""
+        from quart import request, redirect as q_redirect
+        path = request.path
+        # Skip non-page paths
+        if path.startswith(("/static/", "/internal/", "/auth/", "/sx/")):
+            return None
+        # Skip SX expression URLs (already in new format)
+        if path.startswith("/(") or path.startswith("/~"):
+            return None
+        # Skip API/handler paths
+        if "/api/" in path:
+            return None
+        new_url = redirect_old_url(path)
+        if new_url:
+            qs = request.query_string.decode()
+            if qs:
+                new_url += "?" + qs
+            return q_redirect(new_url, 301)
+
     @app.before_request
     async def trailing_slash_redirect():
         from quart import request, redirect
         path = request.path
+        # Skip SX expression URLs — they don't use trailing slashes
+        if "(" in path or path.startswith("/~"):
+            return None
         if (path != "/"
                 and not path.endswith("/")
                 and request.method == "GET"
@@ -128,6 +155,15 @@ def create_app() -> "Quart":
             target = path + "/" + ("?" + qs if qs else "")
             return redirect(target, 301)
 
+    @app.get("/<path:expr>")
+    async def sx_eval_route(expr):
+        """Catch-all: evaluate SX expression URLs."""
+        result = await eval_sx_url(f"/{expr}")
+        if result is None:
+            from quart import abort
+            abort(404)
+        return result
+
     @app.errorhandler(404)
     async def sx_not_found(e):
         from quart import request, make_response