[WIP] Add base64 module for binary-to-text encoding

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>feat(stdlib): add base64 module — base16/32/64/85 encoding</issue_title>
<issue_description>## Summary
Add a base64 module wrapping System.Convert and System.Buffers.Text for binary-to-text encoding, matching Python's base64 module API.

.NET Backing

System.Convert.ToBase64String/FromBase64String, System.Buffers.Text.Base64 for high-perf paths.

Proposed Sharpy API

import base64

encoded: bytes = base64.b64encode(data)           # Standard base64
decoded: bytes = base64.b64decode(encoded)
encoded: bytes = base64.urlsafe_b64encode(data)   # URL-safe variant
encoded: bytes = base64.b32encode(data)            # Base32
encoded: bytes = base64.b16encode(data)            # Base16 (hex)
encoded: bytes = base64.b85encode(data)            # Base85 (Ascii85)
encoded: bytes = base64.a85encode(data)            # Ascii85

Key Decisions

• All functions work with bytes objects (matching Python)
• Standard base64 uses System.Convert; URL-safe variant replaces +/ with -_
• b32/b16 are straightforward; b85/a85 need custom implementation (no .NET built-in)
• Support altchars parameter for b64encode/b64decode

Test Plan

• b64encode/b64decode roundtrip
• urlsafe_b64encode/decode
• b32encode/b32decode
• b16encode/b16decode
• b85encode/b85decode, a85encode/a85decode
• Padding handling
• Error handling for invalid input
• Empty input edge case
  </issue_description>

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

[antonsynd]

Closing empty WIP PR — duplicate of #772 which has the actual implementation.