filesystem: clean up test-to-requirement traceability #552

Apply the test-to-requirement traceability clean-up described in #522 to the Filesystem component (score/filesystem/).

Review every C++ gtest against the current comp_req__filesystem__... needs. Add mandated FullyVerifies or PartiallyVerifies links, TestType, DerivationTechnique, and self-contained Description metadata where a test genuinely verifies a requirement. Reconcile requirement text and test behavior in both directions, and leave non-functional requirements that are verifiable only by inspection without artificial test coverage.