test(render): add scenario tests and test infrastructure

Add 9 scenario tests for component render:
- TestRenderSimpleLocalSpec
- TestRenderWithConfiguredOutputDir (config-provided output dir)
- TestRenderWithOverlayApplied
- TestRenderWithPatchSidecar
- TestRenderStaleCleanup (--force required)
- TestRenderRefusesOverwriteWithoutForce (safety test)
- TestRenderSpecWithUndefinedMacros (golang/gometa via mock)
- TestRenderMultipleComponentsParallel (batch processing)
- TestRenderBrokenSpecWithGoodSpec (partial failure handling)

Test infrastructure additions:
- AddFile(): arbitrary file content in test projects
- WithGitRepo(): git init + commit for synthetic history
- WithRenderedSpecsDir(): set config rendered-specs-dir
- initProjectGitRepo(): uses t.Context() for cancellation

Author: dmcilvaney