Checkboxfixes

[lookfirst]

ugh. this is a few fixes rolled into one. sorry for that, but github pullrequests can be a bit of a pain. i can't figure out how to get just that one commit easily.

[maxatwork]

Closing this as obsolete for the current codebase.

This PR targets the legacy src/form2js.js / src/js2form.js implementation, while the maintained code now lives in the package-based rewrite and the old implementation is kept on the legacy branch.

The nested-table js2form problem from this bundle is already covered on current master by the regression test added in packages/js2form/test/js2form.test.ts (populates nested table fields (issue #23 regression)).

The remaining checkbox/radio changes in this PR do not match current browser behavior or the behavior we intentionally preserve today:

• default checkbox value without an explicit value is browser-native "on"
• unchecked checkboxes are omitted
• radio assignment should match the actual option value, not check every radio in the group
• clearing text inputs to "" is valid and should not be skipped

Current tests already lock the intended behavior in @form2js/dom and @form2js/js2form, so there is nothing left here that we should merge as-is.