Add data contract diffing and schema evolution classification

[parthdongre]

Summary

Add a first-class way to compare two FrameVitals data contracts and classify changes as compatible, warning-level, or breaking.

Why

FrameVitals can already infer and validate contracts, but production pipelines also need to understand how a contract itself changed between versions. Contract diffing would make schema evolution reviewable before a new contract is adopted.

Proposed behavior

Given a previous and candidate contract, report changes such as:

• added/removed columns
• dtype changes
• nullability changes
• tightened/relaxed numeric bounds
• allowed-value changes
• uniqueness changes
• constraint additions/removals

Each change should include a machine-readable category and a compatibility severity.

Acceptance criteria

• A public contract-diff API is defined.
• Common schema/constraint changes receive deterministic classifications.
• Result objects are JSON-serializable and machine-readable.
• CLI support is considered or included if consistent with the existing contract workflow.
• Tests cover additive, relaxed, tightened, and breaking changes.
• Documentation includes at least one CI/schema-migration example.

Priority: P1 — product feature
Suggested labels: enhancement, contracts, data-quality