Deep helpers for object schemas (partial, pick, omit, merge, etc.)

## Objective
Add deep helper methods to the functional API for object schemas to transform types, such as `.partial()`, `.pick<K>()`, `.omit<K>()`, `.merge()`.

### Details
- Helpers should transform the type parameter `T` accordingly while preserving JSON Schema equivalence.
- Document usage, caveats, and migration strategies.

Parent Epic: schema-inference-epic