tar: Preserve PAX mtime nanoseconds

[Copilot]

• Inspect issue Layer has incorrect checksum with Ubuntu 26.04 (resolute) base image -- PAX tar format not round-tripped #290 and PR Initial reimplementation of composefs-c #225 commit 25cbbb1
• Run baseline formatting/clippy/tests; formatting and clippy passed before tests failed on environment-dependent mkcomposefs/fsverity tests
• Apply the tar/PAX fractional mtime extraction from the referenced commit, with the current main branch’s required Stat nanosecond plumbing
• Add focused tests for PAX fractional mtime parsing and splitstream entry metadata
• Run targeted PAX tests and compile workspace tests
• Address validation review feedback on fractional padding and dumpfile nsec parsing
• Run full formatting, clippy, tests, and validation (CodeQL timed out; full tests still fail on pre-existing environment-dependent mkcomposefs/fsverity tests)
• Open a PR with the isolated fix

[cgwalters]

(Sorry for the spam, I was experimenting with copilot again)