Add comprehensive documentation for Canton wallet initialization transactions

[Copilot]

Canton requires a mandatory wallet initialization transaction before wallets can send or receive assets. This party registration process differs fundamentally from traditional address derivation and was previously undocumented.

Changes

Documentation Added

• docs/WALLET_INITIALIZATION.md: Complete technical guide covering:

  • Party-based model and topology transactions
  • Request/response schemas (WalletInitRequest, PreparedParty, WalletInitTransaction)
  • Multi-stage hash computation algorithm (purposes 11, 55)
  • Multi-signature threshold configuration
  • Security validation requirements
  • Integration patterns with BitGo platform

• docs/WALLET_INITIALIZATION_QUICK_REF.md: Reference sheet with:

  • Transaction component tables
  • Builder method signatures
  • Hash computation flow
  • Error troubleshooting guide

• docs/WALLET_INITIALIZATION_FLOW.md: Visual ASCII diagrams illustrating:

  • End-to-end initialization flow
  • Hash algorithm with sorting and length-prefixing
  • Multi-signature workflows
  • Address format breakdown

• README.md: Added documentation links and Canton feature overview

Technical Details

The hash validation is critical for security. Canton uses purpose-prefixed SHA-256:

// Hash each topology transaction (purpose 11)
const rawHashes = topologyTxns.map(tx => sha256Canton(11, tx));

// Sort lexicographically and combine with length prefixes
const combined = sortAndCombineWithLengthPrefix(rawHashes);

// Hash combined buffer (purpose 55) → this is the multiHash you sign
const multiHash = sha256Canton(55, combined);

Key constant corrections:

• PUBLIC_KEY_SPEC: SIGNING_KEY_SPEC_EC_CURVE25519 (not ED25519)
• Purpose prefix: 4-byte big-endian integer (not single byte)

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