Single-source the package and CLI version

Problem

The package manifest declares version 0.2.2, while zig-test --version reports 2.5.0. Hard-coded version strings can drift from release metadata.

Proposed work

• Establish one authoritative version source.
• Expose that version to the CLI at build time.
• Add a test that verifies CLI output matches package or release metadata.
• Document the release version update process.

Acceptance criteria

• The manifest and zig-test --version report the same version.
• Publishing a release requires changing the version in only one place.
• Version output is covered by an automated test.