A full `graphify extract` writes the final graph with `to_json(..., force=True)`,
which bypasses the #479 shrink guard. That is correct for a clean build that
legitimately shrinks (dedup collapse, deleted code), but when this run's
extraction was incomplete — an AST pass crashed, or some semantic chunks failed —
forcing the write lets a partial graph silently overwrite a good complete one.
The build now tracks incompleteness (AST-pass failure, semantic-pass crash, or
succeeded < total chunks) and falls back to the shrink guard (force=False) on an
incomplete run, so a smaller partial graph is refused rather than written. It
exits non-zero before the manifest is written, so the manifest is never stamped
for a graph we declined to write and the next run re-attempts. `--allow-partial`
restores force=True to override intentionally.
Note: the `--no-cluster` raw-dump path writes graph.json directly and has no
shrink guard; this change covers the normal clustered build path only.
