"No default source" when I have a discrete axis map AND split variable fonts defined

I’m currently using FontMake `3.6.0` (the current release).

I have a designspace which is building well with:
- An `ital` axis with a mapping to make it a discrete, binary, on/off axis
- A single variable font, including the full `ital` range

However, if I also add `variable-font` elements that use just one side or the other of the `ital` range (e.g. split roman vs italic VFs), I get an error.

```
INFO:fontmake.font_project:Building variable fonts variable_ttf/AT-NameSansVariable.ttf, variable_ttf/AT-NameSansVariable-Roman.ttf, variable_ttf/AT-NameSansVariable-Italic.ttf
fontmake: Error: In 'source/ArrowType-NameSans-Prep-v011/name_sans-wght_1_1000-opsz_12_96-ital_0_1--w_sparse_supports--unified.designspace': Generating fonts from Designspace failed: No default source; expected default master at Optical Size=8, Weight=700. Found master locations:

```

However, I _do_ have a source at Optical Size=8, Weight=700:

```xml







```

Here’s my `ital` axis and map:

```xml










```

Here’s my variable-fonts element:

```xml























```

If my variable fonts element is only the first font, things work fine, and that font compiles as expected:

```xml









```

Or, if I remove the `mapping` from my `ital` axis, all three variable fonts build as expected:

```xml






```

As far as I understand things, this should all be working, with both the `ital` mapping and all three variable font outputs.

Here’s my full `axes` element, in case that is helpful. (Click to expand)

```xml


Optical Size





Weight






































```

Am I doing something wrong, or missing something?

Thanks so much for any insights here!

Reproduce the failure using the designspace with the discrete `ital` mapping and all three `variable-font` outputs, then compare it with the working single-output and unmapped-axis cases. Trace the handling of default sources and axis subsets; done means all three fonts build successfully while preserving the discrete mapping, with a regression test for the reported configuration.

Written by the indexing model from the issue text.