feat(canopy-snap): self-employment standard deduction parity (PAMMS 3425 / 7 CFR 273.11(a)(2))

Source: tanf-self-employment-net-disregard plan Potential Improvements.

SNAP applies a 40% standard deduction to gross self-employment income per PAMMS 3425. canopy-snap's determine.rs:424 earned-income match path needs review to confirm self_employment vs self_employment_net are handled consistently with the SNAP standard-deduction rule, mirroring the TANF business-expense fix landed under the parent plan.

Acceptance: new plan snap-self-employment-standard-deduction.adoc covering audit + fix; canopy-snap ruleset + canopy-snap aggregation honours the 40% reduction; integration test for a self-employed SNAP applicant with documented gross + net inputs.