Stale detection does not work: Javadoc generation is skipped when it should not

### Affected version

3.12.0

### Bug description

After changing the Javadoc in the Java source files and running `mvn compile javadoc:javadoc` again, Javadoc generation is skipped.

In `maven-javadoc-plugin-stale-data.txt` I see all the command-line arguments and then a bunch of lines like:

```plaintext
C:\path\to\target\reports\apidocs\allclasses-index.html = 1778834612169
```

I assume the right-hand side is a timestamp, but I'm surprised not to see a list of Java source files with timestamps? How can the plugin determine if something has changed if it only has the info in the `maven-javadoc-plugin-stale-data.txt` file?

Environment: Windows 11, JDK 26.0.1, Maven 3.9.15

Reproduce the report on Windows 11 with JDK 26.0.1 by running `mvn compile javadoc:javadoc` after changing a Java source file. Start by inspecting `maven-javadoc-plugin-stale-data.txt` and its recorded command-line arguments and output timestamps; done means Javadoc is regenerated when Java sources change rather than being skipped.

Written by the indexing model from the issue text.