Non-integer axis mapping: Error: Base master not found

With the following axis entry in a DesignSpace file produced by FontLab 8:

```

Weight












```

with gftools 0.9.30, I'm getting the following error:

> Generating fonts from Designspace failed: Base master not found.

Correcting the entry to the following (eliminating fractional mappings so that only integers are used), I get a correct build.

```

Weight













```

Can the mapping contain fractions by spec, or should I raise an issue with the FontLab team?

No source files or tests are named. Reproduce the failure with the provided DesignSpace axis and gftools 0.9.30, then trace how fractional axis mappings are handled during font generation. Done means the behavior is resolved according to the DesignSpace specification, with coverage for the reported fractional mapping case.

Written by the indexing model from the issue text.