add float support to settings.toml and supervisor.get_setting()

[dhalbert]

Add float support for settings.toml, so we can add several new controls for startup values, such as safe-mode delay, BLE startup delay, etc.

This code was developed with careful guiding of Claude Code using the Sonnet 4.6 model. Because this code must be allocation-free, there is some extra code for doing signed numbers beyond calling the basic float number parser.

Because ports/unix does not implement supervisor, a manual test was developed. I ran it and tested it. The original generated test used math.inf and math.nan, which are not available (#10974), and I worked around that.

Also I clarified some comments having to do with SETTINGS_ERR_*.

I fixed a couple of build problems which were unrelated to primary point of the PR.