test(tools): cover signed celestial vertical links

[zoeyrose]

Summary

Add static-validator regression coverage for filename-derived vertical links on celestial maps, including signed coordinates.

Related: atrinik/classic#530

Implementation / behavior

• Exercise a celestial-v1 sky_above linked source whose upper map is discovered from its coordinate suffix.
• Exercise a signed lower coordinate whose upper link is derived while preserving explicit path precedence in the existing fixture.
• Keep authored-map cleanup separate; this PR changes validator tests only.

Validation

• python3 -m unittest tools.tests.test_validate_exits
• python tools/validate.py
• git diff --check

Limitations / follow-up

• This does not remove redundant authored tile metadata; that work remains separately tracked in content issue #261.