Three new proc-macros under aristo::instrument (Inspect derive, expose_pub
attribute, yield_point! function-like) gated by the opt-in aristo_instrument
cargo feature, plus a thread-local runtime hook for fault-injection harnesses.

Slices 36–42 of docs/ROADMAP.md. See CHANGELOG.md for the per-slice rundown
and the slice-41 ADR (docs/decisions/instrument-surface.md) for architecture
+ spec deviations.