forms: the 100-children cap makes explain-form-invalid say no field has an error #423

Found in the 1.0.0 stress pass.

What happens

The forms collector keeps at most 100 children per group (packages/devtools/src/forms.ts:163). In a larger group, a failing field past the cap is dropped, and nothing says so:

• explain-form-invalid answers "No field reports an error" while the form is invalid (forms.ts:810).
• The other form tools and the panel show the group as if it were complete.

Expected

• A capped group says it was cut off: how many children were kept and how many there are in total.
• explain-form-invalid never claims there's no error when the form is invalid. It says the failing field may be past the cap.
• The tool descriptions mention the cap.

How to check

1. Build a form group with 150 controls.
2. Make control 140 invalid.
3. Call explain-form-invalid.