Best-first traversal over a persistent priority queue.

Depth-first traversal that tracks the return from each visit. It yields
visits when they complete (post-order), evaluates bottom-up folds, and
produces the depth-first trace.

Route-context extensions. Their public names and documentation live in
state-space.core.

Checks the options of the public operations when they are called, and
resolves them into the configuration used by the traversal engines.

Depth-first and breadth-first traversal that yields each visit before its
successors. Pending work is kept as lazy chunks of candidates, one chunk
per expanded visit; a stack of chunks gives depth-first order and a queue
gives breadth-first order.

Worklist propagation of joined values through a state space.

Connects the public operations to the traversal engines. Each function
takes the calling function's name for error messages and whether to
return the trace instead of the operation's ordinary results.

Helpers shared by the traversal engines: successor generation, lazy
one-at-a-time mapping, and trace events.