[codex] Add scenario perf adapter

[bill-ph]

Summary

Adds the PR5 scenario perf adapter so perf is a workload step inside the Duckgres Scenario Runner instead of only a standalone harness.

• add tests/scenario/perf with a perf_queries executor that reuses tests/perf/core.QueryRunner, existing perf drivers, and ArtifactSink
• add posthog_frozen_perf.yaml to provision a fresh dev warehouse, create frozen file views, run the existing frozen perf catalog, and deprovision
• write perf artifacts under artifacts/scenario/<run_id>/perf/ while preserving the existing query_results.csv schema
• fail the scenario step when the perf summary records measured query errors so scenario cleanup still runs
• support Flight SNI by dialing DUCKGRES_SCENARIO_FLIGHT_ADDR with the run-specific managed hostname as TLS ServerName
• make scripts/scenario_run.sh --check-env include scenario-specific required_env

Validation

• GOCACHE=/tmp/duckgres-pr5-review-gocache go test ./tests/scenario/... ./tests/perf/...
• just test-scenario
• just lint
• git diff --check

I did not run the live dev frozen perf scenario in this checkout because the real dev scenario API/PG/Flight/S3 environment is not configured here.

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	4	4	0
E2E/journey files	0	0	0
Workflow files	0	0	0

Signals

• Test cases: +7 / -0
• Assertions: +45 / -0
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 0

Coverage risk: neutral or increased

No coverage-reduction warnings detected.