Dispute resolve function

[ayshadogo]

The function:
Auth: Requires admin/arbiter authorization via require_auth()
Validation:
Checks session exists
Validates session status is Disputed
Validates to_payer + to_payee == available_amount
Validates non-negative amounts
Transfers: Performs budget-aware token transfers to payer and payee
State Update: Sets status to Resolved, stores resolved_at, resolver, and note
Event: Emits DisputeResolved { session_id, to_payer, to_payee, fee_total }

closes #79

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