Add regex fallback for WASM isValidAccountName failures

TEMPORARY WORKAROUND for wax WASM thread-safety issue.

The wax WASM module isn't thread-safe - concurrent Server Component
requests calling chain.isValidAccountName() cause memory corruption:
"RuntimeError: memory access out of bounds"

This fix:
- Keeps wax as primary validation method
- Catches WASM errors and falls back to regex validation
- Logs warnings when fallback is used (helps track issue frequency)
- Documents as temporary with TODO and issue references

Once wax#140 is resolved, remove the try/catch and regex fallback.

Related: #758
See also: wax#140