Reconsider checksum delimiter for better text selection ergonomics

[kpiljoong]

Current canonical form:

<prefix>_<payload>[-<checksum>]

Using - as the checksum delimiter has UX issues:

• Double-clicking or using word selection in many UIs often stops at -, so the checksum is excluded from the selection.
• This makes copy/paste of the full ID+checksum error-prone.

Proposal:

• Use _ as the checksum delimiter as well, e.g.:
  • <prefix>_<payload>_<checksum>
• Define semantics based on underscore count:
  • 1 underscore → plain ID (no checksum).
  • 2 underscores → ID with checksum.
  • Any other pattern → invalid.

Acceptance criteria:

• Spec updated with chosen delimiter strategy and rationale.
• Canonical string form and examples updated accordingly.