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.

test/dataset/custom_uid.wren

@@ -0,0 +1,24 @@
+// retoor <retoor@molodetz.nl>
+
+import "dataset" for Dataset
+
+var ds = Dataset.memory()
+var users = ds["users"]
+
+var user = users.insert({
+    "uid": "my-custom-id-123",
+    "name": "Alice"
+})
+
+System.print(user["uid"]) // expect: my-custom-id-123
+System.print(user["name"]) // expect: Alice
+
+var found = users.findOne({"uid": "my-custom-id-123"})
+System.print(found != null) // expect: true
+System.print(found["name"]) // expect: Alice
+
+users.update({"uid": "my-custom-id-123", "name": "Alice Smith"})
+var updated = users.findOne({"uid": "my-custom-id-123"})
+System.print(updated["name"]) // expect: Alice Smith
+
+ds.close()