docs(architecture): add local index diagnostics evidence fabric ADR

[mdheller]

Summary

Adds a proposed SourceOS architecture ADR for a local index, diagnostics, health, trust, and evidence-receipt fabric.

This captures what we learned from the uploaded macOS Spotlight / ecosystem analytics / diagnostic stackshots:

• do not build a monolithic background daemon that mixes indexing, analytics, trust verification, cloud path resolution, logging, and diagnostics;
• split indexing, filesystem event ingestion, trust validation, health observation, and receipts into bounded services;
• require actor/object/operation/policy/receipt/resource-budget evidence for expensive or sensitive actions;
• default cloud, semantic embedding, backup, and sync exposure to explicit signed intent;
• add resource-budget enforcement instead of passive logging.

Files

• docs/architecture/sourceos-local-index-diagnostics-evidence-fabric.md

Follow-up implementation backlog

• sourceos-indexd
• sourceos-fs-eventd
• sourceos-healthd
• sourceos-receiptd
• extractor sandbox contract
• Knowledge & Indexing UI panel
• report/schema canonization in SourceOS-Linux/sourceos-spec

Validation

Documentation-only change. No code validation required in this PR.