Support Base45 (RFC 9285)

[dipu-bd]

Add a Base45 codec for QR-code payloads (EU Digital COVID certificate and similar). Base45 packs 2 bytes into 3 characters over a 45-symbol alphabet.

Architecture fit

• New radix core under lib/src/core/: block-wise radix on 2-byte groups (base-256² → base-45³), with the trailing single byte encoded as 2 chars.
• Codec class in lib/src/codecs/base45.dart (private ctor, static const standard instance), public toBase45/fromBase45 in lib/src/base45.dart with a codec: override, export added to codecs_base.dart.

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

Spec: RFC 9285

Effort: M

Checklist

• Radix core + boundary handling (2-byte and 1-byte tail)
• Generated alphabet table + alphabet-string assertion test
• Six standard test groups incl. RFC 9285 vectors + differential vs base_codecs
• README / CHANGELOG (_next_) / example / test_integration updated
• dart format, dart analyze --fatal-infos, dart test (vm + node) green