Support Base58

[dipu-bd]

Add a Base58 codec — the marquee gap for a crypto-family foundation (Bitcoin / IPFS CID / Ripple / Monero addresses & keys). Base58Check stays in hashlib; this ticket is the plain Base58 codec plus the reusable radix core.

Architecture fit

• Introduces a new radix / bignum core under lib/src/core/ (base-256 ↔ base-58 via repeated division), reused by Base62 and Base45.
• Leading-zero handling: each leading 0x00 byte maps to a leading 1 character and back — the classic Base58 convention. Test this explicitly.
• Codec class in lib/src/codecs/base58.dart (private ctor, static const bitcoin / ripple / flickr instances as appropriate), public toBase58/fromBase58 in lib/src/base58.dart with codec: override, export in codecs_base.dart.

Differential oracle: package:base_codecs (dev-dep).

Spec: https://digitalbazaar.github.io/base58-spec/

Effort: L · Sequence: first codec — establishes the radix core.

Checklist

• Radix core with repeated-division encode / decode
• Leading-zero → leading-1 mapping, tested at boundaries
• Generated alphabet table + alphabet-string assertion
• Six standard test groups incl. differential vs base_codecs
• Web-safe: no 32-bit-shift assumptions in the division loop (node)
• README / CHANGELOG / example / test_integration updated