docs: add docs re current stxo design

[Roasbeef]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @Roasbeef, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the documentation for the Taproot Assets protocol by introducing a detailed guide to the Spent Transaction Output (STXO) proof system. The newly added stxo_design.md comprehensively explains the cryptographic foundations, data structures, and operational lifecycle of STXO proofs. This documentation clarifies how assets are proven to be validly spent and protected against double-spending through the integration of burn keys and a two-level Merkle-Sum Sparse Merkle Tree, providing essential insights for developers and users into the protocol's security and integrity mechanisms.

Highlights

• Comprehensive Documentation: A new stxo_design.md file has been added, providing an in-depth explanation of the Spent Transaction Output (STXO) proof system in Taproot Assets.
• Core STXO Concept: The document details how STXO proofs ensure assets are validly consumed and not double-spent, using minimal asset representations with provably unspendable "burn keys" within the Merkle-Sum Sparse Merkle Tree (MS-SMT) structure.
• Burn Key Cryptography: It elaborates on the deterministic derivation of burn keys from PrevIDs, utilizing a taproot-style tweak of a Nothing-Up-My-Sleeve (NUMS) point to guarantee uniqueness and unspendability.
• Commitment Tree Integration: The documentation explains the two-level MS-SMT architecture, illustrating how STXO assets are managed within a dedicated EmptyGenesisID namespace for efficient insertion, retrieval, and proof generation.
• Lifecycle and Verification: The process of STXO proof creation during asset transfer initiation, the IsTransferRoot decision, and the subsequent generation and verification of inclusion and exclusion proofs are thoroughly described.
• Security Guarantees: The document highlights the security properties provided by STXO proofs, including prevention of double-spending and replay attacks, burn key unspendability, and collision resistance.
• Versioning and Compatibility: It outlines the TransitionV0 and TransitionV1 versions, clarifying that STXO proofs are mandatory for TransitionV1 transfer root assets, and discusses backward compatibility and downgrade mechanisms.
• Special Case Handling: Specific scenarios like genesis asset minting, split transactions, consolidation transfers, and burn transactions are addressed, detailing how STXO proofs are applied or exempted.

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[gemini-code-assist]

Code Review

This pull request adds a comprehensive design document for the STXO proof system. The document is well-structured and detailed. I've provided a few suggestions to improve clarity, fix typos, and correct minor inaccuracies in diagrams and code snippets to make the documentation even better.

[coveralls]

Pull Request Test Coverage Report for Build 18387463468

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+6.9%) to 56.543%

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Files with Coverage Reduction	New Missed Lines	%
tapdb/multiverse.go	2	80.59%
tapdb/sqlc/transfers.sql.go	2	83.33%
universe/supplyverifier/manager.go	2	64.53%
itest/multisig.go	3	97.94%
universe/archive.go	3	81.01%
tapdb/assets_store.go	9	79.74%

Totals
Change from base Build 18359865357:	6.9%
Covered Lines:	63938
Relevant Lines:	113078

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💛 - Coveralls

[GeorgeTsagk]

Read through it, LGTM

I understand that this has been generated with the aid of an agent -- perhaps we could distill it and make it a bit less verbose. Non-blocking though, just personal preference.

[lightninglabs-deploy]

@ffranr: review reminder
@Roasbeef, remember to re-request review from reviewers when ready