feat: add persistent scan_index/url_policy state with --rescan and --reset-url-policy

testdata/one.markdown

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+# Test Data One
+
+- [ok](https://httpbin.org/status/200)
+- [redirect](http://github.com)
+- [broken](https://httpbin.org/status/404)
+- Naked: https://httpbin.org/status/500
+- ![img](https://httpbin.org/image/png)

testdata/three.markdown

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+# Test Data Three
+
+- [parentheses in URL](https://en.wikipedia.org/wiki/Function_(mathematics))
+- [trailing punctuation in sentence] See https://example.org/docs, for details.
+- [image in text should be ignored] text before ![logo](https://example.com/logo.png) text after
+- [mailto should be ignored](mailto:team@example.org)
+- bare www should be ignored: www.example.org

testdata/two.markdown

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+# Test Data Two
+
+- [redirect with title](http://github.com "GitHub redirect")
+- [duplicate redirect](http://github.com)
+- [ftp should be ignored](ftp://speedtest.tele2.net)
+- [query and fragment](https://example.com/docs?lang=de#intro)
+- [inline code URL should not be a markdown link](`https://example.org/code`)
+- autolink angle brackets: <https://example.org/autolink>