Ensure createDataFrame examples are up-to-date and executable via :samples (Korro-validated)

### Problem

`createDataFrame` is one of the primary entry points for users — both in tests and during initial exploration of the library.

However, examples in `createDataFrame.md` are currently located in the `:core` module:
- they rely on internal setup
- they are not validated in a real user environment
- they can become outdated or diverge from actual API usage

As a result, users may copy examples that do not work in practice.

### Expected

Examples for `createDataFrame` should be treated as real, user-facing code and validated accordingly.

### Acceptance criteria

- All `createDataFrame` examples are moved from `:core` to `:samples`
- Examples are integrated with Korro and validated during build
- Examples compile and run in a user-like environment (no internal dependencies)
- Documentation uses these validated examples

### Motivation

- `createDataFrame` is a core onboarding API — users start from it
- Examples must be reliable, executable, and up-to-date
- Using Korro ensures examples stay in sync with the actual API
- Prevents documentation drift and broken onboarding experience before 1.0