Support randomized test order with reproducible seeds #14

Motivation

Randomized execution can expose order-dependent tests, but failures must be reproducible.

Proposed work

• Add options to shuffle suites and tests.
• Accept an explicit seed and generate one when omitted.
• Print the seed at run start and on failure.
• Preserve the exact ordering for a given seed.
• Include the seed in JSON, JUnit properties, and test history.

Acceptance criteria

• Re-running with the printed seed reproduces test order.
• Sequential and parallel modes document their ordering guarantees.
• Unit tests verify stable ordering for a fixed seed.
• Shuffle remains opt-in by default.