Enforce signed UK parity counts, directions, and magnitudes in code

[MaxGhenis]

The signed register says water moved about +0.10 and entity counts moved
person +32 / benunit −12, but the verifier only matches surface, column,
expectation, and entity
(packages/microcosm-build/src/microcosm/build/uk_runtime/signed_differences.py:124-184;
tools/verify_uk_spine_parity.py:124-188). On current main, missing candidate
counts and counts set to 1 both matched the count entry with no unsigned
difference; water moving from 0.776937 to 0.0 also matched the
positive-direction entry. Commit 76e39f9b fixes entity scope but does not
enforce the quantitative facts.

Acceptance criteria:

• Represent and validate exact expected counts or bounded count deltas.
• Represent and validate signed direction and bounded magnitude for share and
  total entries.
• Add adversarial tests for omitted counts, implausible counts, reversed
  direction, and out-of-bound magnitude.

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From the post-merge verification of #747 (round-1 review + fresh reproductions on merged main); full evidence: https://github.com/PolicyEngine/microcosm/blob/review/pr-747-audit/REVIEW-747.md

🤖 Filed by the microcosm owner session with Claude Code