feat(base64-rs): add GNU-compatible base64 command for Windows

Windows has no base64 command; certutil is file-only, adds PEM markers
and cannot be piped. This is a real base64.exe matching GNU coreutils:
same flags (-d, -i, -w), stdin/stdout streaming, exit codes and error
messages, so existing muscle memory and scripts carry over.

Verified against coreutils 9.7 with a 153-case differential test. The
subtle behaviours are reproduced: bytes are emitted incrementally so a
stream that goes bad still writes its valid prefix, unpadded tails are
accepted only when canonically encoded, concatenated streams decode as
one, and -w0 suppresses the trailing newline.

Deliberate deviation: '\r' is skipped when decoding alongside '\n', as
CRLF base64 files are routine on Windows and GNU rejects them.

Zero dependencies, so the build works offline.

base64-rs/.gitignore

@@ -0,0 +1 @@
+/target