Preserve historical proposal outcomes when quorum changes

[mswilkison]

Changing quorum currently recalculates old proposals using the latest numerator. An authorized reduction can revive a proposal that failed only for insufficient quorum; an increase can retroactively defeat a queued proposal. Checkpoint quorum numerators and read them at each proposal's snapshot so both governors preserve historical outcomes while future proposals use the updated requirement.

The existing public ABI, current-numerator getter, authorization, events, bounds, and predeployment query behavior are preserved. The implementation reuses the already-pinned OpenZeppelin Checkpoints library.

Addresses the custom-code counterpart of Dependabot #26, CVE-2022-31198. The operator independently confirmed no qualifying proposal is exploitable today. This is a source correction for future deployments; it does not alter the existing mainnet governor or automatically clear a package-version alert.

Stacked on #188 (codex/187-slither-upgrade, base 20a20d5), which is stacked on #182. Merge in order: #182 → #188 → this PR, retargeting each dependent PR after its base merges. Slither 0.8.0 cannot parse the aliased Checkpoints import; the existing scanner upgrade provides Slither 0.11.6 while retaining the detector gate.

Validation:

• The original implementation fails all three new lifecycle regressions: for-only and for-plus-abstain proposal revival after a reduction, and retroactive defeat after an increase.
• The corrected implementation passes all 388 contract tests, including historical/same-block updates, unchanged authorization, rejected queue/execute of defeated proposals, and successful future proposals under the new quorum.
• Build, typecheck, lint/formatting, local deployment, prepack, and diff checks pass. Both governor ABIs are unchanged.
• Independent candidate review found no concrete bypass or regression.
• CI passes on head a2be670: all 388 contract tests, formatting, deployment dry run, documentation preview, and Slither 0.11.6 (85 contracts, 95 detectors, zero findings). The baseline annotations come from Upgrade Slither CI to 0.11.6 #188; this PR adds no detector exclusions or suppressions.