Add retries, repeated runs, and flaky-test reporting

Closes #7

Summary

• add retry budgets, repeated runs, per-test retry overrides, and attempt history to registered and discovered execution
• classify recovered failures as flaky with optional fail-on-flaky exit behavior
• include attempts in human, JSON, TAP, JUnit, and UI reporting
• define bail, timeout, parallel, exit-code, CLI, config, and programmatic semantics

Verification

• zig fmt --check .
• zig build test -j1 --summary all (173/173 tests)
• zig build
• zig build examples
• zig build-lib -femit-docs src/lib.zig
• zig build -Dtarget=x86_64-windows-gnu