test(api): Official-parameter SCALE-20 engineering green on the public facade

[DecisionNerd]

Problem

The SCALE-20 ComputePool numbers (GU-06-MD-D00) were taken from an in-process AdjacencyGraph::with_test_edges kernel. That is not Official-track engineering green: no GraphForge ingest, no reopen/recount, no measured GSI after load, no one-hop/two-hop LIMIT Cypher.

Objective

Add a thin in-tree reference client that runs Graph500 parameters (SCALE=20, ef=16, undirected Kronecker/R-MAT) through the published Rust facade: generate → bulk ingest → reopen → recount/GSI → LIMIT 1000 one-hop and two-hop. Optional ComputePool 1 vs 4 rank observations on that same project.

Debt / regime

• Debt type: test/proof (scale evidence on the wrong surface)
• Quality regime: A (compute) — counts, GSI grade, LIMIT row bounds, evidence JSON. Timing is hardware-specific, never a CI millisecond gate.

Requirements

• Ingest uses GraphForge::new + publish_bulk_nodes / publish_bulk_edges only (UUIDv7 identities). Not AdjacencyGraph::with_test_edges, not GraphWriter.
• Reopen via a second GraphForge::new(Some(path)) and recount live V/E.
• Grade GSI from those counts (GU from known undirected intent) until feat(api): workspace GSI profiler (GD/GU/Gx grade) #398 ships a profiler.
• Run MATCH (a)-[r]->(b) RETURN b.node_uuid AS id LIMIT 1000 and the two-hop form; both finish with ≤1000 rows.
• Evidence JSON matches docs/reference/scale-evaluation.md field names (schema_version: "1"). Generator identity discloses a bench-local Kronecker, not the pinned graph500 binary.
• Do not set track: official. This is Official-parameter engineering green.
• SCALE-6 smoke is required CI. SCALE-20 is ignored / Makefile opt-in. Not GitHub Actions.
• No Graph500 generator in product crates.
• Graph500 BFS kernel / TEPS is a non-goal (paths(by="bfs") is not that kernel).
• Pairwise neighborhood kernels stay skipped at V=2^20.

Acceptance Criteria

• SCALE-6 public-facade smoke: generate, ingest, reopen, GSI GU-01-XS-*, both LIMIT queries, evidence JSON schema.
• Ignored SCALE-20 run: same pipeline; GSI GU-06-MD-*; evidence written to GF_G500_SCALE20_EVIDENCE_OUT (default under build/).
• Generator disclosure is bench-local Kronecker (name/source/seed), not github.com/graph500/graph500.
• Docs state this is not Official-track and not TEPS.
• Makefile bench-g500-scale20 wraps the ignored release test.
• Product crates do not grow a Graph500 generator.

BDD Completion Scenarios

Scenario: SCALE-6 smoke on the public facade
Given Kronecker parameters SCALE=6 ef=16
When the reference client generates, bulk-ingests, reopens, recounts, and runs both LIMIT 1000 queries
Then V/E match the ingested unique-undirected graph
And GSI is GU-01-XS-*
And both LIMIT queries return ≤1000 rows
And evidence JSON has schema_version "1"

Scenario: SCALE-20 engineering green (manual)
Given Kronecker parameters SCALE=20 ef=16
When the ignored release client runs the same pipeline
Then GSI is GU-06-MD-* after reopen
And one-hop and two-hop LIMIT 1000 finish
And evidence records generate/ingest/reopen/cypher_limit_1hop/cypher_limit_2hop
And track is not official

Non-Goals

• Pinned official graph500 binary
• Graph500 BFS / TEPS
• track: official in harness evidence
• Adding Kronecker to graphforge-exec / graphforge-api libraries
• Pairwise kernels at SCALE 20
• CI wall-clock thresholds

Related Issues

• Related: epic(performance): close M4 embedded performance and scale #335 (M4 epic), test(performance): establish the M4 embedded baseline and entry gate #334 (entry baseline), feat(api): workspace GSI profiler (GD/GU/Gx grade) #398 (GSI profiler), test(performance): reconcile final M4 exit evidence and scale claims #345 (exit evidence)

Open Questions

• None (track stays off the official enum until a pinned Graph500 generator is used)

[DecisionNerd]

Implementation is on test/710-g500-scale20-engineering-green (from current main).

• In-tree public-facade client: crates/graphforge-api/tests/scale_g500_scale20.rs
• SCALE-6 CI smoke passed locally (cargo test -p graphforge-api --test scale_g500_scale20: 3 passed, SCALE-20 ignored)
• SCALE-20 remains #[ignore] / make bench-g500-scale20
• Evidence wrapper graphforge-official-parameter-scale20/1 leaves track null and teps null; generator is bench-local Kronecker, not the pinned graph500 binary
• Docs: docs/development/perf-g500-scale20.md, cross-links from scale-evaluation / GSI

Not yet committed or opened as a PR.