Add Pyright type checking for the public API

[parthdongre]

Summary

Add static type checking for the supported public framevitals API and gradually expand coverage across the package.

Why

FrameVitals ships py.typed and advertises itself as a typed package. The public API should therefore be validated by CI so annotations remain trustworthy as the project evolves.

Scope

• Add Pyright configuration to the repository.
• Start with public entry points and result/config types rather than requiring the entire historical codebase to be strict immediately.
• Add a CI job for type checking.
• Document the local type-check command in CONTRIBUTING.md.
• Establish an incremental path for tightening type coverage over time.

Acceptance criteria

• Pyright is configured and reproducible locally.
• Public APIs exported from framevitals are included in type checking.
• Type-checking failures block relevant PRs.
• Contributor documentation includes the command to run locally.
• Existing intentional dynamic boundaries are documented or narrowly excluded rather than globally ignored.

Priority: P0 — correctness / package maturity
Suggested labels: typing, testing, infrastructure