apr(completion): criteria → issue coverage mapping and gap detection

[JSONbored]

Problem

Approved criteria only mean something if the issue backlog actually covers them. A backlog
that leaves a criterion unaddressed guarantees a project that can never complete.

Scope

• Persist the mapping between each acceptance criterion and the issue(s) intended to
  satisfy it
• Coverage check: every blocking criterion maps to at least one open or completed issue;
  report uncovered criteria and orphaned issues (issues mapping to no criterion)
• Run the check when the backlog changes, not only at intake — issues get added and closed
  mid-project
• Surface the coverage state as part of the project's status

Acceptance

At any point in a project, it is answerable whether the current backlog can satisfy the
approved criteria, and which criteria are unaddressed.

Out of scope

Deciding whether a satisfied issue actually satisfies its criterion (sibling).

[linear-code]

JSO-7080