fix: prevent double-use in coordinator rollback, fix reputation compile error, wire fee netting

[vitalis200]

Summary

closes #1246: rollback() in the coordinator contract could revert an already-Delivered workflow back to Available and refund its payment, letting the same physical blood unit be re-allocated to a different hospital while a copy was already delivered. rollback() now rejects WorkflowStatus::Delivered the same way it already rejects Settled.
closes #1245: the reputation contract called Self::require_admin_auth(...) in four places (submit_rating, record_assignment, flag_fraud, update_from_event) but never defined it, so the crate failed to compile. Implemented require_admin_auth, matching the existing admin-check pattern used by pause/unpause.
closes #1243: FeeStructure::calculate_net_amount was dead code — never called from any production path, despite Payment.amount being documented as "net after fees." Wired it into create_payment so the stored payment amount is actually netted against the fee structure.

Test plan

• cargo check -p reputation-contract --lib — compiles cleanly (previously failed with 4x E0599)
• cargo check --lib (top-level contracts crate) — compiles cleanly
• cargo test --lib test_payments:: / test_protocol_invariants:: (top-level contracts crate) — 39 + 10 tests pass unchanged
• cargo test -p coordinator-contract — added test_rollback_blocked_after_delivery, asserting rollback() now returns InvalidWorkflowState on a Delivered workflow and leaves workflow/unit/payment state untouched; replaces the previous test which asserted the vulnerable behavior and didn't compile (referenced a nonexistent WorkflowStatus::Locked variant)
• Fixed one unrelated compile-blocking typo in coordinator tests (CoordinatorError::EmergencyHalt → EmergencyHalted) that was preventing the coordinator test suite from building at all
• Note: several other pre-existing, unrelated compile issues remain in the repo (duplicate enum discriminant in lifebank-soroban/contracts/payments, missing PartialEq derives in top-level contracts tests, stale test signatures in reputation/coordinator tests) — confirmed via git stash comparison to predate this change and left untouched as out of scope.

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