Fix Ed25519 strictness parity

[OkeyAmy]

Summary

• add shared Ed25519 strictness conformance vectors for Rust and Go
• classify non-canonical Go receipt signatures as SIGNATURE_MALLEABILITY before stdlib verification
• document Rust/Go strictness parity in source-of-truth and docs-site security guidance

Closes #12.

Verification

• go test ./pkg/verify
• cargo test --test test_ed25519
• cargo test
• mdbook build docs-site
• gofmt check on changed Go files
• rustfmt --check tests/test_ed25519.rs

Notes

• cargo fmt --check still reports unrelated pre-existing formatting diffs outside this PR; the changed Rust file passes rustfmt --check.

[amazon-q-developer]

This PR successfully establishes Ed25519 strictness parity between Rust and Go implementations. The changes are well-structured and include:

• Shared conformance vectors for cross-language testing
• Proper signature malleability detection in Go (matching Rust's verify_strict behavior)
• Comprehensive documentation updates
• Extensive test coverage for both implementations

The implementation correctly rejects non-canonical Ed25519 signatures (S >= L) as SIGNATURE_MALLEABILITY before stdlib verification, ensuring consistent security behavior across both languages. No blocking issues found.

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