feat(canopy-enrollment): partial-month retention rule per PAMMS 2415

Source: canopy-enrollment-household-issuances plan Potential Improvements.

Per PAMMS 2415, if the continued-benefits window ends mid-month and the household has already received the full month's issuance, Georgia may allow the household to retain that month's allotment. The current implementation counts the whole-month issuance toward overpayment.

Acceptance: post-processing step subtracts the partial month when the decision falls before a configurable cutoff (jurisdiction.toml); cutoff externalised per ADR-011 with a citations.toml entry; new unit test asserts mid-month boundary behaviour.