DEX contract and policy rules for matching/retiring assets

### Problem description
Environmental market participants need ability to 'reconcile' two sides of the environmental balance sheet, i.e. to match and retire pairs of assets they own.

### Requirements
Introduce a Hedera smart contract with the following functionality
- Standard Registries (SRs) can register their tokens (of specific types - i.e. minted by specific policies) as a side for 'matching pairs' of tokens, specifying rules for what is accepted as the other side of the pair based on
- Extendable list of SRs
- Extendable list of policies and their versions by these SRs (specific numbers, ranges, or 'any')
- Token properties, such as their nominal 'weight' etc
- TBD
- Hedera accounts owning tokens can request to match a number of different tokens, which (after the assessment based on the rules above) would result in their retirement
- Smart contract must automatically verify the trust chain of tokens before executing the retirement operation
- TBD should anything be done in the Guardian Policy Engine?

### Definition of done

- Smart contract is deployed on the testnet
- Documentation covers the usage of the contract for SRs and ordinary market participants

### Acceptance criteria
Retirement of tokens operation is cryptographically verifiable, i.e.
- only valid tokens can be retired
- tokens can only be retired under the rules specified by SRs for **both** sides of the pair
- the event of retirement is visible in the chain history

The payload names no implementation files, tests, or entry points. Start by surveying the Guardian policy, token, and deployment architecture, then clarify the TBD items and contract boundaries with maintainers; done means a testnet deployment, usage documentation for both participant types, and verifiable retirement events satisfying the stated rules.

Written by the indexing model from the issue text.