P19a — saved graph decisions and comments guide

Base 156b790, isolated wt/docs-graph-decisions. Update existing docs/web-ui-search/investigations.md and docs/organizations/service-api.md. Source frontend d25f5a90 (integrated 55944676), backend approved C13 brief 20bb182; final backend serialization fixes must preserve this contract and be ready before publication.

Document saved graph item entry points, human staff decisions, bounded exclusion reasons, explicit restore without validation, reader-specific graph threads and mentions, draft preservation and explicit recovery after conflicts/unknown outcomes. Distinguish saved-case decisions from the free derived evidence view. Explain machine comments without machine validation. Do not promise graph diff, reconciliation or automatic promotion/exclusion propagation.

Write all guide/report Markdown before final typecheck and full Docusaurus build; inspect generated anchors and local links. Independent root quality and p16_backend conformity before coordinator publication. No push or principal edits.