Add Case Studies section

[kevinryu-tob]

Adds a new Case Studies section to the README (between Public Comments and Security Reviews) and a case-studies/ folder with our five finalized case study PDFs:

Client	Focus	Date	Effort (eng-weeks)
Turnkey	TEE and AWS Nitro Enclaves infrastructure	Jun 2026	8
Vanta	Multi-tenant isolation architecture review	Nov – Dec 2025	6
Scopely	Monopoly GO! PRNG fairness design assessment	Dec 2024	2
Hugging Face	Gradio 5.0 pre-release security assessment	Jun – Jul 2024	4
Scroll	zkEVM circuit security reviews	Apr – Sep 2023	45.8

Notes:

• Dates and effort cross-referenced against the existing Security Reviews tables and the underlying review reports (Gradio: June 17 – July 2, 2024; Vanta: November 17 – December 15, 2025, six engineer-weeks total per the report).
• Scroll's 45.8 engineer-weeks comes from the case study itself and matches the sum of the 2023 zkEVM wave engagements.
• Added a 📖 (case study) icon to the Legend.

These will next be surfaced on trailofbits.com/library/ via a follow-up PR to the website repo.

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