The node arena: parsed-tree handles shared across lanekeep's rule-execution engines.

[NodeArena] and [Handle] are how a parsed file's nodes cross into a rule, as opaque integer handles rather than materialized objects (docs/architecture.md §14). That invariant is not specific to any one engine, and neither is the arena: it is built on lanekeep-query and lanekeep-lang alone, with no engine-specific dependency at all.

Why this is its own crate rather than living in lanekeep-js

It used to. lanekeep-js's QuickJS sandbox was the only engine that ran a rule then; lanekeep-wasm's component runtime is the second one now, and both need this exact type: what a node is, and how a handle resolves to one, cannot differ between them without a single run being able to disagree with itself about whether two handles name the same node. Defining it once, here, is what keeps that question from being askable — the alternative, a copy per engine, is exactly the hand-maintained duplication lanekeep's own self-check rules exist to catch elsewhere.

lanekeep-core was the first place this was tried; it does not fit, because lanekeep-query — which the arena needs to run a query against a subtree — already depends on lanekeep-core. A dependency back from core would be a cycle, not a move.

See [nodes] for the arena itself, including why it stores paths from the root rather than borrowed tree_sitter::Node references.