Fix test fixture, README provenance and remaining Windows-only polish #13

Follow-up from #12:

1. TestCompareWindowsOnly (plugin/scripts/godebugdiff_test.go) gives winreadlinkvolume Changed: 26. The real value is 23, so the change gets the URL go.dev/doc/godebug#go-126 instead of #go-123. To reproduce: comment out c.WindowsOnly = isWindowsOnly(k) in compare() and run the test; the failure message prints URL:https://go.dev/doc/godebug#go-126. Use 23. The test also ignores the second value compare() returns and never checks Before, After or Package. Asserting them costs nothing.
2. The WindowsOnly field comment now says "Windows-specific setting", but isWindowsOnly only checks the "win" name prefix. Make the comment match the rule, or derive the mark from an explicit list.
3. examples/README.md line 12 still says the real outputs were made with Go 1.27.3, and the new line 24 says cobra.txt was redone with Go 1.27.4. Put the toolchain for each file in one place, for example a column in the table.
4. Item 3 of Regenerate saved examples and test that compare() sets windowsOnly #11 is still open, although test: cover Windows classification and refresh saved outputs #12 closes Regenerate saved examples and test that compare() sets windowsOnly #11: the "No binary imports the package, so no effect" section still adds "(Windows only)". To reproduce: run from 1.22 to 1.23 on a module whose only binary is an empty func main() {}. Also, windowsOnly still uses omitempty while goCommand is always emitted, and the report does not tell Linux-only services that they can ignore the pins for Windows-only settings.

GillMacedo