Base45 encoding already implemented per RFC 9285

[Copilot]

Base45 encoding for QR code binary data (RFC 9285) is fully implemented in the codebase.

Implementation Status

• Core: Base45.cs provides encode/decode with span-based, stream-based, and async APIs
• Alphabet: RFC 9285 compliant 45-character set: 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ $%*+-./:"
• Multibase: Integrated with encoding prefix 'R' (0x52)
• Tests: 62 tests covering RFC test vectors, edge cases, streams, and async operations
• Docs: Listed in README features, benchmarks included

Usage

using SimpleBase;
using System.Text;

var data = Encoding.UTF8.GetBytes("base-45");
string encoded = Base45.Default.Encode(data);  // "UJCLQE7W581"

byte[] decoded = Base45.Default.Decode(encoded);

All 1,730 repository tests pass including Base45 test suite.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add Base45</issue_title>
<issue_description>Used for binary encoding in QR codes. https://datatracker.ietf.org/doc/rfc9285/</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ssg/SimpleBase#76

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