NFI physics debt: issue 1228 slice 3 unit-contract carriers

[neuron7xLab]

Goal

Close the highest-priority remaining unit-contract debt after PR #1353 without granting score credit from planning text.

Scope

Issue #1228 tasks:

• 18. dt unit row
• 19. Unit contract artifact
• 20. Unit contract smoke test
• 21. Homogeneity test link
• 22. Unit pass/fail summary

Rules

• Artifact-only verdict.
• Score remains 81.5.
• No target interval claim.
• No final physics validation claim.
• Static unit labels remain bookkeeping contracts, not physics-invariant witnesses.
• PR must include a commit acceptor binding all changed files.

Required carriers

• docs/physics/units_table.md explicit dt row.
• tests/physics/test_dimensional_homogeneity.py as smoke/homogeneity link.
• artifacts/physics_validation/issue_1228_slice3_unit_contract_verdict.json as pass/fail summary.
• .claude/commit_acceptors/issue-1228-slice3-unit-contract.yaml as governance carrier.