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/hard_delete.wren

@@ -0,0 +1,29 @@
+// retoor <retoor@molodetz.nl>
+
+import "dataset" for Dataset
+
+var ds = Dataset.memory()
+var users = ds["users"]
+
+var user = users.insert({"name": "Alice"})
+var uid = user["uid"]
+
+System.print(users.count()) // expect: 1
+
+users.delete(uid)
+System.print(users.count()) // expect: 0
+
+var softDeleted = ds.db.query("SELECT * FROM users WHERE uid = ?", [uid])
+System.print(softDeleted.count) // expect: 1
+System.print(softDeleted[0]["deleted_at"] != null) // expect: true
+
+var user2 = users.insert({"name": "Bob"})
+var uid2 = user2["uid"]
+
+System.print(users.hardDelete(uid2)) // expect: true
+var hardDeleted = ds.db.query("SELECT * FROM users WHERE uid = ?", [uid2])
+System.print(hardDeleted.count) // expect: 0
+
+System.print(users.hardDelete("nonexistent")) // expect: false
+
+ds.close()