`graphify extract <root> --out <dir>` reduced every node's `source_file` to a
bare filename, so a graph.json could no longer be resolved back to files on
disk by joining source_file onto the scan root.
--out passes the output dir as cache_root to relocate the cache, but that value
also anchored relativization. Every scanned file then failed relative_to(root),
fell through to the #1899 out-of-root fallback, tripped its `updepth > 3`
walk-up guard -- written for a stray ProjectReference, not a whole corpus -- and
collapsed to a basename. On Windows an --out on another drive hit the
cross-drive branch and basenamed unconditionally, which is what the reporter
saw: 0 of ~120k source_files kept a separator. The directory survived only in
the node id slug, lossily (`.`, `/`, `\`, `-`, spaces all map to `_`), and no
other field carried it -- origin_file is stripped (#1516) and the export has no
file table -- so 0% of nodes resolved.
extract() now takes an explicit `root` anchor for source_file/ids/symbol
resolution, which the CLI pins to the scan root independent of where the cache
lives. This completes the cache/anchor decoupling #1774 started and matches
build(root=target), which already anchored on the scan root -- extract was the
lone component keying off --out.
cache_root keeps its fallback-anchor role, so callers that pass the scan root as
cache_root (watch, the no---out CLI path, tests) are unchanged, as are cache
location and out-of-root portability (#1899).
