refactor(base58): replace mr-tron/base58 with in-tree long-division encoder

[sonicfromnewyoke]

Problem

the base58 package's fast paths only covered 32/64-byte inputs, so every other length (instruction data, Base58.MarshalJSON, Data.String) had to go through the external github.com/mr-tron/base58 module - a needless dependency, and one with a latent buffer-sizing bug that panics on all-zero input
this change pulls the long-division algorithm in-tree behind a unified base58.Encode entry point and drops the external dependency

Summary of Changes

• add base58.Encode(buf []byte) string dispatcher: 32/64-byte fast paths + vendored long-division fallback (encodeVariable) with the mr-tron buffer-sizing bug fixed
• drop the mr-tron/base58 module dependency; rewire all callers (Base58, Data, Hash, Signature, PublicKey, PrivateKey) to base58.Encode
• update tests/fuzzers to cross-validate the matrix-multiply fast paths against the in-tree encodeVariable - no existing exported signatures changed (gorelease -> v1.20.0, additive only)

[HealthyBuilder]

LGTM — removing the mr-tron dependency makes sense