Support Base62 and Base36

[dipu-bd]

Add Base62 and Base36 for URL shorteners, short DB IDs, and compact tokens.

Architecture fit

• Base36 is a thin wrapper over the existing BigIntCodec — bytes → BigInt → radix-36 string, and back. Cheap ergonomic win.
• Base62 needs a leading-zero convention (like Base58) so that leading 0x00 bytes survive the BigInt round-trip; document the chosen convention.
• Public toBase62/fromBase62, toBase36/fromBase36 with codec: overrides; exports in codecs_base.dart.

Differential oracle: cross-check Base36 against BigInt.toRadixString(36) for the numeric core; Base62 leading-zero behavior needs vendored vectors.

Effort: S–M

Checklist

• Base36 wrapper on BigIntCodec
• Base62 with documented leading-zero convention
• Alphabet-string assertions for both
• Six standard test groups incl. differential/known-answer vectors
• README / CHANGELOG / example / test_integration updated
• Format / analyze / test (vm + node) green