Add Field component with validation, autowiring, and composite control support

[banditburai]

Summary

• Adds the Field component family (Field, FieldSet, FieldLegend, FieldGroup, FieldContent, FieldLabel, FieldTitle, FieldDescription, FieldSeparator, FieldError) for accessible form field composition
• Deep autowiring walks nested children to find a single text-like control, enabling wrappers like input + copy button to still auto-wire IDs and aria attributes
• Checkbox gains field_control param to explicitly opt into Field autowiring
• validate= parameter auto-creates signals, wires FieldError, and sets data-invalid reactively
• validate_event parameter controls when validation fires (default: "input")
• Full docs page with 7 examples: basic, orientation, states, form submission, composite control, fieldset, and separator

Test plan

• All 419 tests pass
• ruff check and ruff format --check pass
• star sort --check passes (Tailwind class ordering)
• pyright passes with 0 errors
• Registry index regenerated with updated checksums