In build.go, build-haks now does conservative chunk reuse instead of
wiping all generated HAKs up front. It plans chunks as before, computes
a deterministic per-chunk content_hash, reuses an existing .hak only
when the prior manifest entry and on-disk archive still match, rebuilds
only changed chunks, and deletes only stale generated HAKs that fall out
of the new plan. I added coverage for unchanged reuse and selective
rebuilds in pipeline_test.go, and go test ./... passes in sow-tools.
archvillainette
bf87144688