fix: validate checksum input before computing checksum

[kpiljoong]

Summary

Validate the base ID before computing an optional checksum during parsing so malformed checksum-bearing input returns normal parse errors instead of reaching checksum helper panics.

This change:

• defers checksum verification until after prefix, payload length, and Base32 validation
• keeps checksum mismatch behavior unchanged for otherwise valid IDs
• adds regression coverage for malformed checksum inputs that previously could panic

Tests:

• go test ./...

Refs #11