feat: add merge_all_wren utility, strutil demo, and 7 new API manual pages

Add WrenFileReader class in apps/merge_all_wren.wren for recursive .wren file discovery and content printing. Introduce example/strutil_demo.wren demonstrating case conversion, hex encoding, SHA256 hashing, string repetition, padding, HTML/JSON escaping, and URL encoding/decoding. Create manual/api pages for argparse, dataset, html, markdown, uuid, wdantic, and web modules. Update manual/api/index.html sidebar and module count from 21 to 28, adding links and cards for the new modules.

apps/merge_all_wren.wren

@@ -0,0 +1,31 @@
+import "io" for File, Directory
+
+class WrenFileReader {
+  static listWrenFiles(dir) {
+    var files = []
+    var entries = Directory.list(dir)
+    for (entry in entries) {
+      var fullPath = "%(dir)/%(entry)"
+      if (Directory.exists(fullPath)) {
+        files.addAll(listWrenFiles(fullPath))
+      } else if (entry.endsWith(".wren")) {
+        files.add(fullPath)
+      }
+    }
+    return files
+  }
+
+  static readFiles(files) {
+    for (file in files) {
+      System.print("--- %(file) ---")
+      System.print(File.read(file))
+    }
+  }
+
+  static main() {
+    var wrenFiles = listWrenFiles(".")
+    readFiles(wrenFiles)
+  }
+}
+
+WrenFileReader.main()