[P1 governance] Parity exemption debt can grow against merge base under existing issue numbers

[bhelm]

Confirmed gap

The merge-base ratchet protects removed shards and new ledger findings, but it does not diff individual exempt entries. A PR can add arbitrary declaration exemptions under an existing broad issue such as #17 and remain green, so exemption debt is not actually bounded.

Acceptance

• Diff exemption entries against merge base.
• Every newly added exemption requires a new exact issue and non-empty exact reason, with bounded governance that prevents a pre-existing umbrella issue from acting as a blank cheque.
• Removal/debt reduction remains allowed.
• Red-first tests cover added exemption, reused pre-existing issue, malformed reason, and valid new issue-bound addition.

Tracks tooling only; no production behavior change.