feat: establish NeuroSleep safety foundation

[ruvnet]

Summary

• record ADR 051 and its research-only NeuroSleep boundary
• enforce ADR 007 minima: 5 observations for trends, 20 aligned pairs for correlation, and 10 observations per change-point segment
• serialize insufficient trends as typed abstentions rather than flat/stable
• add electrophysiology provenance and downstream regression coverage

Safety

• no NeuroSleep ingestion or user-facing claim path is enabled
• safety gates are excluded from MetaHarness/Darwin mutation surfaces
• existing loose neural adapter remains legacy behavior

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace
• git diff --check