feat: add optional package-level concurrency

[jeffatgametime]

Adds an opt-in RunWithOptions path for package-level concurrency while preserving Run() as the serial default for existing callers.

Changes:

• Adds RunOptions{PackageConcurrency} and RunWithOptions.
• Factors per-package execution into local result builders so concurrent runs don't append to shared slices.
• Preserves deterministic package result ordering.
• Adds a race-covered unit test for bounded package fan-out.

Validation:

• go test ./...
• go test -race ./...

Downstream consumer:

• gametimesf/api-tests branch docs-parallel-package-test-execution wires this into hosted acceptance runs via TEST_PACKAGE_CONCURRENCY.