feat: enable mediorum core writes

[raymondjacobson]

Summary

• switch OPENAUDIO_MEDIORUM_CORE_WRITES_ENABLED default from false to true
• leave the legacy CRUDR peer transport active so older nodes still receive metadata writes during the rollout

Compatibility

• Backward compatible: writes are dual-delivered because CRUDR remains active, so pre-PR1 nodes still receive ops through the old path.
• Forward compatible: PR1+ nodes receive the same op ULIDs through Core and apply them idempotently if CRUDR already delivered them.

Verification

• go test ./pkg/core/server -run 'Test(IsValidSignedTransaction|ValidateMediorumOperationShape)'
• go test ./pkg/mediorum/crudr -run '^$'
• go test -c ./pkg/mediorum/server -o /tmp/mediorum-server-pr2.test
• git diff --check