feat(stacks): add CAIP-19 asset specification

[whoabuddy]

Summary

This PR adds a comprehensive CAIP-19 specification for Stacks token assets, addressing the feedback from #68.

• Adds stacks/caip19.md with support for SIP-010 (fungible) and SIP-009 (non-fungible) tokens
• Updates stacks/README.md to reference CAIP-19

Relationship to #68

This is an updated implementation that addresses all review feedback from #68:

Original Concern	Resolution
Undocumented .contract syntax	Clear explanation of three components: {address}.{contract}.{token-name}
Incomplete documentation	Complete Rationale, Syntax, RegEx, Semantics, Examples sections
Case sensitivity	Dedicated subsection explaining address normalization vs case-sensitive contract/token names

Key Design Decisions

Asset Reference Format:

{address}.{contract}.{token-name}

• Uses . as separator (CAIP-19 compliant, since : is not permitted in asset_reference)
• Includes token name for precise identification (Stacks contracts can define multiple tokens)
• Documents relationship to Clarity's native :: syntax

Namespaces:

Namespace	Description
sip010	Fungible tokens (SIP-010, analogous to ERC-20)
sip009	Non-fungible tokens (SIP-009, analogous to ERC-721)

Examples (verified on-chain):

# sBTC fungible token
stacks:1/sip010:SM3VDXK3WZZSA84XXFKAFAF15NNZX32CTSG82JFQ4.sbtc-token.sbtc-token

# Megapont Ape Club NFT #4
stacks:1/sip009:SP3D6PV2ACBPEKYJTCMH7HEN02KP87QSP8KTEH335.megapont-ape-club-nft.Megapont-Ape-Club/4

Test plan

• Syntax follows CAIP-19 specification (no colons in asset_reference)
• Examples verified against Hiro API for on-chain accuracy
• Asset reference lengths within 128 character limit
• All reference links valid

References

• Supersedes Create stacks caip19.md #68
• Related: SIP-009 (NFT), SIP-010 (FT) standards

🤖 Generated with Claude Code

[whoabuddy]

cc @kyranjamie as well!