docs: refresh NestJS case study with measured two-pass remediation

[Ayush7614]

Summary

• Restructures website/docs/case-studies/nestjs.md around measured remediation on NestJS revision cee51af using CVE Lite v1.25.0 (2026-06-24)
• Adds a publication-grade Before vs After table: lockfile baseline 51 → 50 → 47 across two passes (fastify@5.8.5, then mocha@12.0.0-beta-4)
• Documents what cleared after each pass, preserves honest remaining-risk discussion (47 findings still present), and aligns usage-aware triage (51 → 10 with --only-used)
• Updates scan verification metadata and reproduction steps for a local NestJS clone (no in-repo fixture)

Fixes #158

Test plan

• Verified markdown renders in docs site structure
• Numbers tied to live scans on pinned revision (cee51af)
• Two-pass Fix Journey matches measured deltas (pass 1: direct fastify; pass 2: Mocha parent chain)
• Remaining risk section reflects post-pass-2 counts without zero-CVE framing

[Ayush7614]

cc: @sonukapoor

[sonukapoor]

Clean update — numbers are internally consistent throughout (51→50→47 two-pass, --only-used 51→10), the remaining risk section is honest (4 critical still present after both passes), and the fix journey documents the peer-dependency friction accurately. Good work on the measured passes.