FINERACT-2455: Working Capital Loan - Write-Off

[alberto-art3ch]

Description

Adds Write-Off and Undo Write-Off for Working Capital Loans, matching cumulative/progressive loans.

Write-off is terminal: zeroes outstanding balances and closes the loan as CLOSED_WRITTEN_OFF; undo reopens it to ACTIVE and restores the balance. Incoming parameters follow progressive loans.

Accounting (accrual): Dr LOSSES_WRITTEN_OFF (total) · Cr LOAN_PORTFOLIO / FEES_RECEIVABLE / PENALTIES_RECEIVABLE per portion (no interest leg). Undo reverses via offsetting mirrors.

Includes: domain/state machine + dedicated write-off domain service, transaction/handlers/API/validator wiring, accounting WRITEOFF case, business events, DB migration 0063 (0062 reserved for the in-review Charge-Off PR), integration + E2E tests, and a fineract-doc chapter.

FINERACT-2455

Checklist

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[galovics]

Migration and permission wiring are solid, WorkingCapitalLoanWriteOffDomainService is clean. One real gap:

// TODO(D3): confirm the backdating rule. The ticket says "not supporting backdated transactions"; progressive
// loans allow a floor at the last user transaction date. Add the agreed date constraint here once confirmed.

The validator only checks "not null" and "not in the future" for the write-off date - the actual backdating restriction the ticket calls for is admittedly unresolved and unimplemented. As it stands, a write-off can be backdated to any past date (before other transactions, before disbursement, whatever), and the write-off domain service doesn't guard against it either (zeroes outstanding unconditionally). For a terminal, balance-zeroing operation this seems worth nailing down before merge rather than after - self-acknowledged TODOs on the actual validation rule the PR is supposed to implement are the kind of thing I'd want resolved, not shipped.