Add regex fallback for WASM isValidAccountName failures

Related: #758 See also: wax#140

Problem

WASM memory corruption under concurrent load:

WaxError: RuntimeError: memory access out of bounds
    at nn.isValidAccountName

Solution (TEMPORARY WORKAROUND)

Keep wax as primary validation, add regex fallback when WASM fails:

try {
  const chain = await getChain();
  return chain.isValidAccountName(accountName);
} catch (error) {
  logger.warn({ err: error, accountName }, 'WASM isValidAccountName failed, using regex fallback');
  return isUsernameValidRegex(accountName);
}

This is intentionally a temporary fix until wax thread-safety is resolved (wax#140).

What this does

• Tries wax WASM validation first (preserves intended behavior)
• On WASM error, logs warning and falls back to regex
• Logging helps track how often the issue occurs
• Well-documented with TODO markers and issue references

Easy to revert

Once wax#140 is fixed, simply:

1. Remove the try/catch wrapper
2. Remove the isUsernameValidRegex function
3. Remove the logger import if unused

Regex validation rules (matches Hive consensus)

• 3-16 characters
• Lowercase letters, numbers, dots, hyphens only
• Must start with a letter
• Cannot end with hyphen or dot
• Each segment must start with a letter
• No consecutive dots or hyphens

Test plan

• Deploy and verify errors stop (fallback kicks in)
• Check logs for "WASM isValidAccountName failed" warnings
• Valid usernames work: gtg, alice.bob, test-user
• Invalid usernames rejected: 123test, test-, ab