Use a relative target for the latest log symlink

[Old-Ding]

Description

Create the latest log symlink with the run directory basename as its target. Because the link and run directory are siblings, the relative target resolves identically in place and remains valid when the parent log directory is mounted at a different absolute path.

Add a platform-independent regression test that verifies the target is relative and resolves back to the run directory.

Fixes #978

Is this user-facing behavior change?

Yes. latest continues to resolve to the newest run directory, but os.readlink() now returns a relative basename instead of an absolute path. This keeps the link usable across host and container mount paths.

Did you use Generative AI?

Yes. OpenAI Codex (GPT-5) was used to trace the logging path, implement the one-line behavior change, add the regression test, and review the diff. The contributor reviewed and locally validated the final change.

Additional Information

Local validation:

• pytest launch/test/launch/test_logging.py: 21 passed
• pytest launch/test/launch: 280 passed
• pytest launch/test/test_flake8.py: 1 passed

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