fix Use field defaults from schema in v1

[danielcardoso5]

Fixes default values not being taken into account when initializing a form like v0 did. Supports passing default values for base fields, nested fields and group-array items.

With the schema from #249:

before	after

---

Note

Medium Risk
Changes initial form state and conditional visibility for all consumers relying on missing defaults; behavior differs from v0 for explicit falsy initial values.

Overview
Restores v0-style default seeding so createHeadlessForm applies JSON Schema default values before the first field build, fixing conditionals (e.g. if/then) that depend on a defaulted field not resolving until after handleValidation.

A new fillDefaults helper walks the base schema: object properties, nested objects, and existing array items only. It sets default when the value is undefined—explicit falsy values like null are kept (documented as a v0 vs v1 migration difference). Defaults inside conditional sub-schemas are not applied. initialValues are not mutated.

MIGRATING.md adds a breaking-change note on default semantics; tests cover init-time conditionals, overrides, nested and group-array cases.

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[dragidavid]

Sorry for the delay @danielcardoso5