feat: expose package runner primitives

[jeffatgametime]

Adds opt-in package-level primitives without introducing any Temporal dependency into testy.

Changes:

• Adds PackageSpec, ListPackages, RunPackage, and BuildSuiteResult.
• Keeps Run() serial by default and RunWithOptions behavior unchanged.
• Lets downstream orchestrators run packages externally and assemble a suite result.

Validation:

• go test ./...
• go test -race ./...

Downstream consumer:

• gametimesf/api-tests branch temporal-test-execution uses these primitives for Temporal activity orchestration.