Introduce fuzz and snapshot tests for tag parsing #21

Summary

Bolster the tag parser with fuzz testing and snapshot coverage across all supported file types to prevent regressions like the AWS Lambda image rewrite bug.

Why

Tag parsing mistakes are high-impact. Fuzzing and exhaustive snapshots reduce the chance of subtle errors slipping through when adding new grammars or registries.

What needs to happen

• Add fuzz tests (e.g. using fast-check or similar) targeting Docker tag parsing and version detection utilities.
• Create snapshot tests for each supported file type, verifying both positive matches and skip cases.
• Integrate the fuzz run into CI with sensible runtime limits.
• Document how to run the fuzz tests locally.

References

• Roadmap quality item: "Fuzz tests for tag parsing".
• Related bug: Detect invalid Python patch versions in Docker image tags #8

Acceptance criteria

• Fuzz tests run in CI and catch malformed tag handling.
• Snapshot fixtures cover existing and newly-supported file types.
• Documentation explains the test strategy and how to execute it.