Fix duplicated content in checksum section of spec

[kpiljoong]

The "checksum" section in the spec currently duplicates content from the ID generation section (likely a copy-paste error).

Actions:

• Remove the duplicate text.
• Replace it with a concise description of checksum-specific behavior:
  • Checksum input: `"_".
  • Algorithm: Bech32 polymod (20 bits).
  • Encoding: Crockford Base32 into 4 characters.
  • How to validate on parse.

Acceptance criteria:

• Checksum section focuses only on checksum semantics and computation.
• No redundant "ID generation" content in that section.