Harden qet-bench publication release candidate

[f-gueguen]

Summary

• add the vacuum-investigation roadmap to the qet-bench docs set
• reconcile package, citation, manuscript, and roadmap metadata around the v0.2.0 review target
• add a root repository README for public-review discoverability
• add a ReScience-oriented reproducibility manuscript source
• document local Python 3.11/3.12 validation, optional Qiskit/Aer smoke results, and the hosted-CI billing/quota blocker

Local validation

• Python 3.11.6 base CI mirror: install, pytest, reproduce_benchmarks.py, make_all_figures.py; 40 passed, 1 skipped
• Python 3.12.8 with optional Qiskit/Aer: install, pytest, reproduce_benchmarks.py, make_all_figures.py, run_qiskit_bridge.py; 39 passed, 2 skipped
• Pandoc citation smoke builds: paper.md and rescience.md passed, wrote /tmp/qet-bench-manuscript.html and /tmp/qet-bench-rescience.html

Hosted CI

• GitHub Actions jobs are currently blocked before runner start by account billing/spending limit. Representative blocked run: https://github.com/lucent-lab/qet-bench/actions/runs/24507161016

Notes

• root docs/ archive material is ignored intentionally; publishable docs are under qet-bench/docs/
• optional Qiskit bridge CSV remains generated under ignored qet-bench/results/ unless explicitly archived later