test(gfql): pin ms-column temporal decline accurately

[lmeyerov]

Summary

• Fix the sub-resolution temporal test so its fixture row is actually 2020 rather than 1970 after an unintended us cast.
• Assert that Polars declines a +1us bound on a ms Datetime column instead of rounding it.
• Add exact-ms positive and no-match controls.
• Test-only change; no runtime or API redesign.

Evidence

• pytest graphistry/tests/compute/gfql/test_epoch_repair_regressions.py: 22 passed.
• ruff check graphistry/tests/compute/gfql/test_epoch_repair_regressions.py: clean.

Gates

Hosted CI must be terminal on this exact head. No merge or release is requested in this PR.

• Exact-head terminal hosted evidence: CodeQL run 39002159807 and CI Tests run 39002159811; RTD check SUCCESS.