Add durable D&D combat turn artifacts

internal/modules/dnd/codec/combatturns/testdata/dnd_combat_turns.v1.json

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+{"combat_turns":[{"actor":"Aria","turn_kind":"turn","round":1,"actions":[{"category":"attack","declaration":"Aria swings her sword","targets":["wight"],"resolution":"The wight is hit."}],"summary":"Aria attacks the wight.","source_refs":[{"source_id":"session-alpha","start_unit_id":1,"end_unit_id":2}]}]}