The CalibrationSampler joined PSKR midpoints against hrrr_profiles via
nearest-within-window, but post-Phase-3 Stream C nothing bulk-populates
the grid — hrrr_profiles only sees per-QSO point fetches from the Rust
hrrr-point-worker. Result: 0 / 16,042 prod samples had HRRR data.
Producer pattern: when a cell has no profile in the lookahead window,
enqueue a row into hrrr_fetch_tasks at the cell's HRRR-grid-rounded
midpoint. The Rust worker drains it; the next 5-min rolling-window
pass UPSERTs the same sample with non-NULL weather fields. Idempotent
with the existing on-conflict design — no extra worker, no churn on
cells already covered, multiple bands sharing a midpoint dedupe to a
single fetch via Enum.uniq + jsonb point-union in HrrrPointEnqueuer.