Built-in graph artifact family + source-backed graph retrieval

[marklubin]

Problem

Entity and relation artifacts are useful enough to model directly in Synix, but graph retrieval must always terminate in source-backed evidence rather than orphan synthetic nodes.

Scope

In scope:

• entity and relation artifact schemas
• graph indexing over checkpoint-scoped banks
• graph-informed expansion back to source evidence
• runtime exposure through the standard tool API

Out of scope:

• external graph databases
• research-heavy temporal graph features not needed for the first milestone

Dependencies

• Depends on Built-in chunk artifact family #83
• Depends on Python-local runtime/tool API for snapshot-scoped banks #82

Acceptance Criteria

• Graph artifacts are typed and provenance-complete
• Graph lookups resolve back to source-backed refs
• Graph builds are checkpoint-scoped and reusable across reruns

Required Automated E2E Test

Build graph artifacts over checkpoint-scoped source data, mount the bank, perform graph expansion through the runtime, and verify returned paths resolve back to underlying evidence.

Docs Updates Required

• built-in graph family reference
• graph retrieval semantics
• provenance expectations for graph-derived refs

Demo Or Template Follow-On

Record a future graph-memory-investigation demo or template extension that showcases graph retrieval resolving back to source evidence.