fix: compare only observed baseline sections

[DivyamTalwar]

What and why

Closes #100.

Unavailable or missing collector sections could be interpreted as measured zeros or empty sets, creating false connection drops, standby losses, new-query entries and archiver events. Require observable section provenance at both ends while preserving genuinely measured empty results and positive evidence in legacy snapshots. A retained positive connection gauge remains usable when only rates are unavailable.

Verification

Verified commit: 9811ed1faab48344389ba5c5a31687df272e887d.

• Regressions fail on unchanged base behavior for missing current observations, missing baselines and empty legacy sections.
• The real SQLite Save/reopen/offline-diff path reproduces false changes and passes with the fix.
• Affected diff/CLI race suites, clean committed-HEAD bash scripts/gate.sh, and go test -race ./... pass.
• Controls preserve genuine losses/additions, measured zero, legacy positive values, rate-reset connection gauges, deterministic output and input immutability.

Scope, risk and rollback

No schema, SQL, dependency or migration changes. This suppresses comparisons unsupported by observations, not genuine measurements. It is distinct from #64 (cross-reset comparability), #69 (standby multiplicity) and #87 (stored scalar extraction), though the diff edits may need reconciliation as those land. No live PostgreSQL failure is claimed. Revert the commit to restore the former comparison rules.

Checklist

• scripts/gate.sh passes on committed HEAD; full race suite passes.
• Read-only, privacy and deterministic-finding invariants preserved.
• No new finding or model.Context JSON field; catalog/model schema unchanged.