Fix SNAP expected contribution rounding per 7 CFR 273.10(e)

[hua7450]

Summary

Fixes the rounding in snap_expected_contribution, which previously computed floor(net_income) * 0.3 with no rounding of the product.

Fixes #9312

Legal basis

• 7 CFR 273.10(e)(1)(ii)(A): net income is rounded to the nearest dollar — 1–49 cents down, 50–99 cents up (half-up, so np.floor(x + 0.5) rather than np.round's half-to-even).
• 7 CFR 273.10(e)(2)(ii)(A)(1): 30 percent of net income is rounded up to the next higher dollar — equivalent to rounding the allotment down to the nearest lower whole dollar (7 U.S.C. 2017(a)).
• The product is rounded to cents before np.ceil so float32 noise (e.g. 50 * 0.3 = 15.0000009) cannot push an exact dollar amount up to the next dollar.

Tests

• New snap_normal_allotment_basis_of_issuance.yaml (13 cases): 3-person allotments at net-income boundaries validated against the FNS Basis of Coupon/EBT Issuance table (Oct 1, 2025), including net income $3.49 vs $3.50 half-up boundary and the exact-multiple $50/$51 rows.
• New snap_categorical_eligibility_ceiling.yaml (3 cases): categorically eligible units above the issuance-table maximum (2-person gets the minimum allotment, 3-person gets $0 but stays flagged eligible per 7 CFR 273.10(e)(2)(iii) not converting to denial in the model).
• Updated 16 existing expectations whose old values carried fractional cents (e.g. 230.40 → 230, 288.9 → 288) — artifacts of the missing rounding.

Known CI failures (intentional)

~30 SNAP pins in tests/policy/baseline/partners/** shift by $1–$11/year under the corrected rounding. Per repo policy those partner contract files are not edited in this PR; the partner shard will fail until that change is separately approved.

Test plan

• 16 new tests pass locally; the 4 updated files pass locally (policyengine-core test ... -c policyengine_us)
• CI (partner shard expected to fail, see above)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (57743a2) to head (8df8c50).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #9318   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         1    -2     
  Lines           65        14   -51     
=========================================
- Hits            65        14   -51     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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