[Dijkstra] Move `Coin-TokenAlgebra` to a shared location

[williamdemeo]

Background

src/Ledger/Dijkstra/Foreign/ExternalStructures.lagda.md imports Coin-TokenAlgebra from Ledger.Conway.Specification.TokenAlgebra.Coin. The instance has no Conway-specific content, so the cross-era source dependency is incidental.

Goal

Move Coin-TokenAlgebra (and the Ledger.Conway.Specification.TokenAlgebra.Coin module if appropriate) to a shared Ledger.Core.* location, or define a parallel Ledger.Dijkstra.Specification.TokenAlgebra.Coin that re-exports the same instance. Update Ledger.Conway.Foreign.ExternalStructures and Ledger.Dijkstra.Foreign.ExternalStructures to import from the new location.