Ensure graph benchmark exits after reporting

[SNComrade]

Summary

• flush stdout/stderr and exit after the ADR-130 graph benchmark prints its report
• prevent imported graph/embedding modules from keeping CI alive until the workflow timeout after successful output

Verification

• node --check scripts/benchmark-graph.mjs

Context

Recent upstream main V3 CI runs reached the ADR-130 benchmark summary, then the graph benchmark job stayed alive until GitHub canceled the workflow around the 40 minute timeout. This keeps the benchmark's existing report behavior and makes the process termination explicit after output is emitted.

[SNComrade]

Local verification while upstream fork Actions are waiting on approval:

• node --check scripts/benchmark-graph.mjs passed.
• corepack pnpm install --frozen-lockfile in v3/ completed.
• corepack pnpm -r build in v3/ completed.
• node scripts/benchmark-graph.mjs completed under a 120s guard on Windows and exited cleanly.

Benchmark output reached Targets: 6/6 met locally. The run still prints the existing Transformers.js/sharp fallback message before using mock embeddings, matching the already-observed CI dependency noise; it did not keep the process alive after reporting.

[ruvnet]

Reviewing; will return after PR #2139 merges. The graph benchmark exit fix is targeted and looks like a real bug — ensuring the benchmark process exits cleanly after reporting. Will fast-track review once the quality sweep baseline is established.