Headless crawl paths are not exportable

Pure headless builds a crawl graph but does not expose a replayable path from the entrypoint to each location. Audit-style work and clean-session rewalks need that path as a first-class object, not only a DOT dump of the graph.

Add a Path type (entry, target, ordered actions), PathFromRoot / AllPathsFromRoot on the graph, Paths() on the crawler, and paths.json when diagnostics are on.