# Reachability lattice

Reachability is modeled as a deterministic lattice with explicit unknowns.
Signals emits per-target states and a fact-level score that is stable under
replay.

Current v0 model (buckets)
- Buckets: entrypoint, direct, runtime, unknown, unreachable.
- Scores are confidence * bucket weight, clamped to 0.0-1.0.
- Unknowns pressure penalizes the fact-level score to avoid false safety.

Unknowns pressure
- pressure = unknowns / (targets + unknowns)
- score = avg(target scores) * (1 - min(pressure, ceiling))

Lattice v1 (design direction)
- States: unknown, staticallyReachable, staticallyUnreachable,
  runtimeObserved, runtimeUnobserved, confirmedReachable,
  confirmedUnreachable, contested.
- Joins are monotonic; contested absorbs conflicts.
- Policy should treat unknown and contested as under investigation.

Evidence requirements
- Reachability facts include graph hashes, path references, and runtime hit refs.
- Evidence transitions record previous state and supporting inputs.

Policy guidance
- Do not assert not_affected without high-confidence reachability evidence.
- Unknown or contested states should surface as under_investigation.

Related references
- docs/reachability/lattice.md
- docs/signals/unknowns-registry.md