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/edge_cases.wren

@@ -0,0 +1,38 @@
+// retoor <retoor@molodetz.nl>
+
+import "dataset" for Dataset
+
+var ds = Dataset.memory()
+var users = ds["users"]
+
+var emptyAll = users.all()
+System.print(emptyAll.count) // expect: 0
+
+System.print(users.count()) // expect: 0
+
+var user = users.insert({"name": "Alice", "score": 100})
+
+var notFound = users.findOne({"name": "Nobody"})
+System.print(notFound == null) // expect: true
+
+var emptyFind = users.find({"name": "Nobody"})
+System.print(emptyFind.count) // expect: 0
+System.print(emptyFind is List) // expect: true
+var uid = user["uid"]
+
+var changes1 = users.update({"uid": uid, "score": 200})
+System.print(changes1) // expect: 1
+
+var changes2 = users.update({"uid": uid, "score": 300, "level": 5})
+System.print(changes2) // expect: 1
+
+var updated = users.findOne({"uid": uid})
+System.print(updated["score"]) // expect: 300
+System.print(updated["level"]) // expect: 5
+
+var fakeChanges = users.update({"uid": "nonexistent-uid", "score": 999})
+System.print(fakeChanges) // expect: 0
+
+System.print(users.delete("nonexistent-uid")) // expect: false
+
+ds.close()