Optimized base62 implementation using OTP-style dispatch and
bit-syntax accumulators, with full test coverage (CT + PropEr +
A/B benchmark against frozen legacy baseline).

Requires OTP 27 or later (uses EEP-48 -moduledoc/-doc markdown).

Speedup over the 1.0.x baseline: 2.6-2.8x encode, 1.9-2.3x decode.

See CHANGELOG.md for the full change list and BENCHMARK.md for
the full A/B table.