Completes step 1 of docs/plan/derived-blobs.md. The chunk-level
`actual_ref_count` recompute was two correlated subqueries written
inline; it now sums the registered reference sources instead, so
`blobs_consistency` and `dedup_gc` answer "what references this hash"
from one place. If they ever diverged the sweep would bless counts the
collector disagrees with — and the collector wins, destructively.
No behaviour change: the generated expression is the same legacy-files
term (guarded by NOT EXISTS) plus the same manifests-citing-this-chunk
term, and a golden test pins the whole statement byte-for-byte.
Built once at construction, like the reap statement, so the sweep runs
a fixed query per page rather than assembling SQL inside the loop. The
builder refuses an empty registry rather than emitting a query where
every blob looks unreferenced and the entire table reports
refcount_mismatch; there is a test.
DI now constructs one registry and hands the same instance to both
consumers — `DedupService::reference_registry()` is what
`BlobsConsistencyCheck` receives, so agreement is structural rather
than a convention someone has to maintain.
The long comment explaining the single-chunk double-count trap moved
from the query site to the builder's doc comment, where the NOT EXISTS
guard it describes actually lives.
fmt, clippy --all-features --all-targets and the 17 affected unit tests
all clean.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Edouard Vanbelle