Core types and execution engine for lanekeep.

File walking, query evaluation, the facts pipeline, violations, and the Rule trait.

This crate owns the contract every other crate is written against. Rule is treated as public API that happens not to be published: no built-in rule may reach past it into walker internals or cache state, because that boundary is what keeps future rule sources additive. See docs/architecture.md §14.

What is here so far

Rule identity, severity, source locations, rule cards, violations with their canonical ordering, and the facts a per-file pass hands to the reduce phase.

These types are foundational in a specific sense: they are what appears in JSON output, in cache entries, and in suppression comments users type by hand. Getting them wrong is expensive in a way that getting the walker wrong is not, because only these are visible from outside.