net: share exact aggregate diagnostic counting rules

[Patrick W. Healy (phealy)]

Scope

Stacked on #770. Extracts reusable exact diagnostic counting rules so summary collection can preserve existing route mismatch and normalized peer-link health semantics without building a full outbound snapshot. No behavior/default switch in this foundation.

138 changed LOC.

Validation

Shared status and controller tests passed after stacking; focused race/parity tests, formatting/lint and builds passed during implementation.

Lightweight status draft stack

Base: #745. Each layer targets the preceding branch and remains draft.

#746 -> #747 -> #748 -> #750 -> #751 -> #752 -> #753 -> #754 -> #755 -> #756 -> #757 -> #758 -> #759 -> #760 -> #761 -> #762 -> #763 -> #764 -> #765 -> #766 -> #767 -> #768 -> #769 -> #770 -> #771 -> #772 -> #773 -> #774 -> #775 -> #776 -> #777 -> #778 -> #779 -> #780 -> #781 -> #782 -> #783 -> #784 -> #785 -> #786 -> #787 -> #788 -> #789 -> #790 -> #791 -> #792 -> #793 -> #794 -> #795 -> #796 -> #797 -> #798 -> #799

[Patrick W. Healy (phealy)]

This PR was mistaken churn from splitting the lightweight-status work into unnecessarily small PRs. That was my mistake, not a requirement from the user.

Its changes are preserved in #812 within the user-approved, consolidated 12-PR draft stack. Closing this superseded PR; the implementation is not being discarded, and nothing has been merged into main.