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.

Impact

Benefits shift weakly downward only (EC_new >= EC_old always): at most ~$1.10/month (up to ~$0.99 from rounding 30% up, plus up to ~$0.30 when net income rounds upward), i.e. $1–$13 per year on annual totals.

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 the 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 baseline expectations whose old values carried fractional cents (e.g. 230.40 → 230) — artifacts of the missing rounding.

Partner contract tests (updated with approval)

64 SNAP pins under tests/policy/baseline/partners/analytics_coverage/ shift by $1–$13/year under the corrected rounding and were updated with Ziming's approval:

• edge_cases (8 files, 31 pins): pins updated and every arithmetic comment rewritten to the new chain (round net half-up → 30% rounded up), e.g. EC 337.20 → ceil(1,124 × 0.3) = 338.
• signatures (ca/federal, 33 pins): raw model-output pins updated (these were masked in the first CI run because the job aborted on the edge-case failures).

The change responds directly to the API partner's own rounding report, which serves as partner notice.

Test plan

• 16 new tests pass locally
• Updated baseline files pass locally
• Updated partner files pass locally (29 + 18 + 46 cases)
• CI fully green

🤖 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     
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- Hits            65        14   -51     

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

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