docs(test): test & bench coverage strategy (levels × aspects + interop + risk transfer)

Captures #21: a two-axis matrix (unit/P2P/E2E/endpoint × functionality/performance/dependability/security/accessibility/interoperability/maintainability), per-aspect coverage, consolidated interoperability + risk-transfer sections, today-vs-blocked, and a prioritised rollout. Flags currently-broken/false-green gates (VRAM eviction accounting, SPDX-header head-20, licence single-source, scorecard score-parse, missing SBOM) and the two headline gaps: interp-vs-wasm differential oracle + no-GC-vs-GC churn bench. Reshapes #12.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath