> Fossil...would force me to commit the proverbial 500-line blob all at once
Nope.
If it were me doing such a thing as you describe, I'd start the work on a feature branch. If I'm working on that repo with other active developers, this lets them see what I'm up to and possibly help; and if not help, then at least be aware about where my head's at, so they can better predict what's likely to land on the shared working branch later.
If I got to a point where only part of the branch needed to be applied, I could cherrypick those individual changes, either down to the parent branch or up to a higher-level feature branch.
All of this happens in public, with the work fully recorded, so someone doesn't have to reconstruct the development history after the fact later.
This mode of development helps keep your project's bus factor above 1.
You can commit code that doesn't compile onto a feature branch so people can see what you're up to, I guess, but I don't see that helping with bisecting later, and I wouldn't expect the commit messages to be useful.
To be clear, my typical approach is certainly to commit every time I return to a working state. But in more experimental modes, I often reach that the long-way-around and have ended up with multiple semantic changes I wish to break apart for study.
Unless I'm missing something and Fossil has gained the ability to cherry-pick selective lines from a commit.
Nope.
If it were me doing such a thing as you describe, I'd start the work on a feature branch. If I'm working on that repo with other active developers, this lets them see what I'm up to and possibly help; and if not help, then at least be aware about where my head's at, so they can better predict what's likely to land on the shared working branch later.
If I got to a point where only part of the branch needed to be applied, I could cherrypick those individual changes, either down to the parent branch or up to a higher-level feature branch.
All of this happens in public, with the work fully recorded, so someone doesn't have to reconstruct the development history after the fact later.
This mode of development helps keep your project's bus factor above 1.