feat: add registry config

[matteojug-stacks]

Add smart contract registry in spox config.

Testing information

Add some unit tests on the parsing, the whole flow was also tested locally with spox registry aware + front end (coming soon™)

[Copilot]

Pull request overview

Adds support for configuring an optional Stacks smart-contract registry in the spox configuration, including parsing and validation.

Changes:

• Add registry_contract (optional QualifiedContractIdentifier) to Settings with a custom deserializer.
• Add validation + new SpoxConfigError variant to require stacks config when registry_contract is set.
• Extend default config documentation and add unit tests for parsing the registry contract.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/config/serialization.rs	Adds contract_deserializer_option to parse optional qualified contract IDs.
src/config/mod.rs	Extends Settings with registry_contract, adds validation, and adds parsing tests.
src/config/error.rs	Introduces MissingStacksConfig validation error.
src/config/default.toml	Documents the new registry_contract config/env var.

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[djordon-stacks]

Looks good