Review follow-up on #14 (second round). The first-observation probes reused the
correlation queries' scalar filters but not their joins, so `first_observation`
could predate every row the fit can use — measured on prod:

    source              probe before   probe after (HRRR/band_map matched)
    contacts            1991-05-04     2016-09-17
    beacon_measurements 2026-08-10     2026-08-25

`resolve_holdout` decides the fallback by comparing the cutoff against that
value, so a source whose matched rows all sit inside the rolling window got the
holdout applied, the `fit_cutoff > source_first_obs` invariant held against a
population the fit never had, no note fired, and the fit came back empty — the
silent state this PR exists to remove. At a 4-week window today the old probe
applies the beacon holdout and its fit returns 0 bands / 0 rows, while 8,133
raw beacon measurements sit inside that window.

The join predicates are now written once (`CONTACTS_HRRR_MATCH`,
`BEACON_HRRR_MATCH`, `BEACON_BAND_BIN_MATCH`, `BAND_MAP_VALUES`) and
interpolated into both the correlation query and its probe, so the two cannot
drift; the probes use `EXISTS` rather than the full join, since a `MIN` needs
the population and not the matched rows. PSKR needs no join — its query reads
`pskr_calibration_samples` directly — and says so.

Verified against prod: contacts and pskr still resolve the 8-week cutoff
(2026-07-25) and beacon still falls back with its note, now off the joinable
first observation; at 4 weeks the beacon holdout is declined instead of applied
to an empty fit; a full `--dry-run` reproduces the same 14 merged bands
(3 PSKR, 11 contacts, 0 beacon) as before the SQL refactor, confirming the
shared predicates are the ones the fit already used.

@@ -194,0 +255,4 @@
+            "no rows match the source filters; fitting the full (empty) "
+            "corpus with no holdout"
+        )
+    if cutoff <= source_first_obs:

@@ -337,2 +425,4 @@
 
 
+# First-observation probes: one `MIN(...)` per source, filtered like the
+# corresponding correlation query so each rolling cutoff is resolved against

@@ -339,0 +435,4 @@
+"""
+
+CONTACTS_FIRST_OBS_SQL = """
+SELECT min(qso_timestamp)

@@ -194,0 +249,4 @@
+        raise ValueError(f"holdout weeks must be >= 0, got {weeks}")
+    if now.tzinfo is None:
+        now = now.replace(tzinfo=dt.timezone.utc)
+    cutoff = now - dt.timedelta(weeks=weeks)

@@ -194,0 +263,4 @@
+            "with no holdout"
+        )
+
+    assert cutoff > source_first_obs, "holdout cutoff would empty the fit window"