fix(Base58): preserve leading zeros for zero-valued and empty inputs

[spokodev]

Problem

Base58 drops a leading 1 for all-zero inputs and throws on empty input, so round-trips are broken for zero-valued payloads:

Base58.fromHex('0x00')    // ''      (should be '1')
Base58.fromHex('0x0000')  // '1'     (should be '11')
Base58.fromHex('0x')      // throws  (should be '')
Base58.toHex('1')         // '0x000' (should be '0x00')
Base58.toHex('')          // '0x0'   (should be '0x')

Inputs with a non-zero tail were already correct (e.g. 0x0000287fb4cd → 1111233QC4), which is why the existing leading-zero vectors passed and this went unnoticed. It only surfaces when the value portion is entirely zero.

Impact: any all-zero or empty Base58 payload round-trips to the wrong byte length and is rejected/misread by other Base58 tools (bs58, ethers). Empty input throws an uncaught Cannot convert 0x to a BigInt instead of returning ''.

Cause

src/core/internal/base58.ts (encode): the leading-zero loop runs while bytes.length > 1, so the final zero byte is never counted (n zero bytes produce n−1 1s), and BigInt('0x') throws on empty input.

src/core/Base58.ts (decode): integer.toString(16) appends a stray 0 nibble when the decoded value is 0n.

Fix

• Count every leading zero byte (> 0 instead of > 1).
• Treat empty hex as 0n instead of throwing.
• Emit no body nibble when the decoded integer is 0 (the zero bytes come entirely from the leading-1 count).

Non-zero inputs are untouched: both loops already stop at the first non-zero byte, and the decode body is unchanged for non-zero values.

Spec

The Base58 spec referenced in the Base58 docs (and Base58Check) maps each leading 0x00 byte to exactly one 1. Output verified to match bs58 and ethers across all-zero, leading-zero, empty and random inputs.

Tests

Added regression cases for the all-zero and empty paths on fromHex / toHex / toBytes. They fail on main and pass with the fix; the existing suite is unchanged (12/12 green).

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