Add SYNC_GUARDS thresholds so a partial/degenerate upstream response never
triggers a destructive operation:
- skip stale category deletion when the fetched tree is suspiciously small
vs the existing SDS category count
- skip delist detection unless both leaf-category and seen-product counts
are healthy
Verified: 77 tests pass; live SDS returns 226 categories (guard off),
incident-case ratios (2/226, 2/2) are correctly blocked.
yeuimu
aed9afef92