fix: reject mixed-case bech32 addresses

[tolga-tom-nook]

Summary

• Reject mixed-case Bech32/SegWit addresses in isAddressValid() before checksum normalization.
• Keep all-lowercase and all-uppercase Bech32 addresses valid, but reject mixed-case strings per BIP173.
• Add regression coverage and update the distributed dist/functions/address.js entry point used by the package main.

RED proof

With the new regression added before the fix, npm test failed in Address functions: isAddressValid:

AssertionError: expected true to equal false
-true
+false

The failing address was a mixed-case BGL Bech32 address: Bgl1QW508D6QEJXTDG4Y5R3ZARVARY0C5XW7K0FY5A3.

GREEN proof

npm test

Result: 68 passing.

node - <<'NODE'
const assert = require('assert');
const jsbgl = require('./');
(async () => {
  await jsbgl.asyncInit(global);
  assert.strictEqual(isAddressValid('bgl1qw508d6qejxtdg4y5r3zarvary0c5xw7k0fy5a3'), true);
  assert.strictEqual(isAddressValid('BGL1QW508D6QEJXTDG4Y5R3ZARVARY0C5XW7K0FY5A3'), true);
  assert.strictEqual(isAddressValid('Bgl1QW508D6QEJXTDG4Y5R3ZARVARY0C5XW7K0FY5A3'), false);
  console.log('dist/package bech32 mixed-case regression ok');
})();
NODE

Result: dist/package bech32 mixed-case regression ok.

git diff --check

Result: clean.

Bounty context

This is submitted as a Bitgesell improvement/bugfix PR for the public bounty/improvement program:

• BGL (Bitgesell) Bounty/Improvement Program ($50000 budget) bitgesell#81
• BGL PR bounty hunt ($10000 overall budget) bitgesell#39

[tolga-tom-nook]

Pushed a small test-hardening follow-up (8074a86) to cover the same Bech32 mixed-case rule on testnet addresses too.

Validation rerun locally:

npm test
# 68 passing

node dist/package regression probe
# mainnet/testnet bech32 case regression ok

git diff --check
# clean

This keeps the implementation unchanged and adds explicit regression coverage that:

• all-lowercase and all-uppercase mainnet/testnet Bech32 remain valid;
• mixed-case mainnet/testnet Bech32 are rejected before checksum normalization.