Add testing docs and PR CI workflow

[carlosrivera]

Problem

• No centralized testing documentation and no PR CI that runs cargo check / cargo test.

Goals

• Add testing documentation (unit tests, coverage, and TUI smoke checklist).
• Add GitHub Actions workflow to run cargo check and cargo test on PRs.

Acceptance

• docs/testing.md exists and is linked from docs/README.md and README.md.
• PR workflow runs on pull_request and executes cargo check + cargo test.

[carlosrivera]

Implemented in PR #7. Added testing docs, linked from README/docs index, added PR CI workflow, and expanded unit tests to raise coverage.