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/markdown/safe_mode.wren

@@ -0,0 +1,21 @@
+// retoor <retoor@molodetz.nl>
+
+import "markdown" for Markdown
+
+var unsafe = Markdown.toHtml("<script>alert('xss')</script>")
+System.print(unsafe.contains("<script>")) // expect: true
+
+var safe = Markdown.toHtml("<script>alert('xss')</script>", {"safeMode": true})
+System.print(safe.contains("<script>")) // expect: false
+System.print(safe.contains("&lt;script&gt;")) // expect: true
+
+var safeCode = Markdown.toHtml("```\n<div>test</div>\n```", {"safeMode": true})
+System.print(safeCode.contains("<div>")) // expect: false
+System.print(safeCode.contains("&lt;div&gt;")) // expect: true
+
+var safeInline = Markdown.toHtml("Text with <b>html</b> inside", {"safeMode": true})
+System.print(safeInline.contains("<b>")) // expect: false
+System.print(safeInline.contains("&lt;b&gt;")) // expect: true
+
+var ampersand = Markdown.toHtml("A & B", {"safeMode": true})
+System.print(ampersand.contains("&amp;")) // expect: true