docs(casebook): ecosystem audit edition — 33 verified findings across 71 repos

[roy-tong]

What

Adds the ecosystem-audit edition to the measurement casebook: over 2026-09-06/07 we deep-read 71 repositories (usage tools, observability platforms, eval frameworks, gateways, pricing sources) and filed 33 evidence-backed usage-accounting findings. The new section summarizes the five hazard classes with confirmed-instance counts and links every claim to the public PR/issue.

Honesty rules kept

• States explicitly what the audit does not establish (no adoption, no endorsement, most audited repos passed clean — promptfoo, Portkey model data, tokscale are named as passing).
• One accepted artifact to date (swarm #145 → PR #146, maintainer-credited); all other items listed as open/unreviewed.
• No private logs; all reproductions are synthetic fixtures from the conformance pack.

Why this belongs in the repo

The casebook is the product demo: the same taxonomy and fixture discipline that drive the conformance pack found quantified, verified defects that the targets' own test suites missed. Ends with a pointer to the fixture shapes for tool authors — no AgentMeasure runtime required.