feat(graph): POST /graph/project for pre-typed entity/relation projection

[RichardHightower]

Summary

Implements #235: a projection ingestion path so research-graph (and any other projector) can write pre-typed entities and relations into the property graph with no LLM/AST extraction.

This is Gap A / Phase B from docs/plans/research-graph-make-it-real.md. Projector logic stays in the consumer; Agent Brain only accepts explicit facts.

API

POST /graph/project
{
  "entities":  [{"type": "okf:Claim", "id": "claim.1", "properties": {}}],
  "relations": [{"src": "finding.1", "predicate": "asserts", "dst": "claim.1"}],
  "source_tag": "research-graph",
  "replace": false
}

• Upsert — re-projecting the same id updates in place.
• replace=true / DELETE /graph/project?source_tag= — delete-by-tag so destroy-and-rebuild is deterministic and does not wipe SCHEMA-01 extraction output.
• Namespaced types (okf:Claim) and extra snake_case predicates (asserts, evidenced_by) are accepted alongside SCHEMA-01/03. Invented un-namespaced types still 400.
• GET /graph/entity/okf:Finding/{id} accepts the same vocabulary so the typed spine is traversable.
• Writes go through spawn isolation (project_isolated), same SIGSEGV in kuzu::NodeTable::lookupPK during sustained GraphRAG indexing (v10.0.7) #178 contract as build_from_documents_isolated. A kuzu-native crash is 503 kuzu_unavailable, not a dead server.
• Additive and gated by ENABLE_GRAPH_INDEX / graphrag.enabled. No extractor is invoked.

Tests

• API: tests/unit/api/test_graph_project_endpoint.py plus namespaced-type coverage on GET /graph/entity
• Store: tests/unit/storage/test_graph_project.py (simple backend: spine, idempotent reproject, replace, tag isolation)
• Isolation: tests/indexing/test_graph_project_isolation.py (SIGSEGV-class child exit)

Verified locally against SimplePropertyGraphStore: Finding→asserts→Claim→evidenced_by→Evidence round-trips, delete-by-tag spares untagged SCHEMA-01 nodes, replace rebuild converges.

Closes #235.