feat: add SEP-53 message signing and verification

[tomerweller]

Summary

Closes #2345

Add support for signing and verifying arbitrary messages per SEP-53.

New Commands

stellar message sign - Sign a message using SEP-53 specification

# Sign with an identity
stellar message sign "Hello, World!" --sign-with-key alice

# Sign with a secret key directly
stellar message sign "Hello, World!" --sign-with-key SC36...

# Sign binary data (base64 input)
stellar message sign --base64 "SGVsbG8sIFdvcmxkIQ==" --sign-with-key alice

# Read message from stdin
echo "Hello" | stellar message sign --sign-with-key alice

stellar message verify - Verify a SEP-53 signed message

# Verify with a public key
stellar message verify "Hello, World!" \
  --signature "fO5dbYhXUhBMhe6..." \
  --public-key GABC...

# Verify using an identity
stellar message verify "Hello, World!" \
  --signature "fO5dbYhXUhBMhe6..." \
  --public-key alice

Features

• Sign messages with local keys, seed phrases, or secure store
• Support for both UTF-8 text and binary (base64) input
• Signature output in base64 format
• Identity lookup for both signing and verification

Implementation

Follows the SEP-53 specification:

1. Prepend "Stellar Signed Message:\n" prefix to message
2. SHA-256 hash the prefixed payload
3. ed25519 sign the hash
4. Return 64-byte signature as base64

Limitations

• Ledger hardware wallet signing is not yet supported (returns error)
• Lab signing is not supported (no way to return signature)

Test Plan

• Unit tests for signing with various message types (ASCII, UTF-8, binary)
• Unit tests for verification with valid/invalid signatures
• Unit tests for SEP-53 prefix correctness
• Integration with existing identity/key management

---

🤖 Generated with Claude Code