Document SNAP work-registration sanction income treatment and durations

[DTrim99]

Follow-up to #8862 / #8961, addressing #8968.

The three sanction semantics in #8968:

1. Income treatment — already correct on main. A member sanctioned under the general work requirements (is_snap_work_registration_noncompliant) has is_snap_prorated_income_member = false, so their income is counted in full per 7 CFR 273.11(c)(1) — proration (273.11(c)(2)) applies only to ABAWD time-limit ineligibles — while still being excluded from the SNAP unit. This PR adds a regression test locking that behavior (the issue's acceptance criterion).
2. Head-of-household household sanction (7 CFR 273.7(f)(5)) — deferred. It's a state option, and no state adopter is documented. An empty state-election parameter would add an inert, untested branch to the critical is_snap_eligible path, so I've left it out with a note rather than ship dead logic. Easy to add later behind a populated state list.
3. Durations — documented. The flag is a per-month status with no duration logic; the 273.7(f)(2) minimum sanction periods (1/3/6 months, state options up to permanent) are the caller's responsibility. Documented on the variable.

No behavior change; baseline results are unchanged. is_snap_prorated_income_member suite 9/9.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8326a14) to head (4c8581a).
⚠️ Report is 53 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%            
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  Files           12         1    -11     
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- Hits           161         9   -152     

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

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