feat(ft): optional false_trigger_reason ("offset" etc.) as an FT subtype

A reason records *why* an event is a false trigger. It is optional (plain
FT flags still record no reason) and is a subtype of the FT flag: setting a
reason implies false_trigger=1, and the reason is cleared whenever FT ends
up 0 (confirm-real, clear-FT, set_false_trigger(false)). Twin propagation
carries the reason to the histogram/waveform twin alongside the FT flag.

New nullable `false_trigger_reason TEXT` column (schema + _migrate ADD
COLUMN only — not the Migration-1 rebuild). 7 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDXjZCr4RqT2U3QvMDhgzf

sfm/database.py

@@ -82,6 +82,7 @@ CREATE TABLE IF NOT EXISTS events (
     record_type             TEXT,                   -- "single_shot" | "continuous"
     false_trigger           INTEGER NOT NULL DEFAULT 0,  -- 0=no, 1=yes (manual flag)
     reviewed_real           INTEGER NOT NULL DEFAULT 0,  -- 0=no, 1=operator-confirmed real (mutually exclusive with false_trigger)
+    false_trigger_reason    TEXT,                   -- optional FT cause ("offset", ...); NULL = none. Only meaningful when false_trigger=1.
     blastware_filename      TEXT,                   -- event file within waveform store; extension is per-event (AB0T encodes timestamp)
     blastware_filesize      INTEGER,                -- bytes; NULL if no event file saved
     a5_pickle_filename      TEXT,                   -- "<filename>.a5.pkl" sidecar
@@ -234,6 +235,7 @@ class SeismoDb:
             ("shape_offset_pre",      "REAL"),
             ("shape_offset_spread",   "REAL"),
             ("reviewed_real",         "INTEGER NOT NULL DEFAULT 0"),
+            ("false_trigger_reason",  "TEXT"),
         ):
             if col not in existing_cols:
                 log.info("_migrate: events ADD COLUMN %s %s", col, ddl)
@@ -713,9 +715,9 @@ class SeismoDb:
 
     def propagate_review_to_twins(self, event_id: str, *, window_seconds: int | None = None) -> list[str]:
         """
-        Copy this event's `false_trigger`/`reviewed_real` columns onto each
-        of its histogram/waveform twins (see `find_twins`), so flagging one
-        twin flags both.  Returns the list of twin ids updated.
+        Copy this event's `false_trigger`/`reviewed_real`/`false_trigger_reason`
+        columns onto each of its histogram/waveform twins (see `find_twins`), so
+        flagging one twin flags both.  Returns the list of twin ids updated.
 
         ``window_seconds`` is accepted for backward compatibility but ignored;
         twin matching is now interval-based (see `find_twins`).
@@ -725,12 +727,16 @@ class SeismoDb:
             return []
         ft = 1 if row.get("false_trigger") else 0
         real = 1 if row.get("reviewed_real") else 0
+        # The reason is a subtype of the FT flag — carry it only when the source
+        # is actually a false trigger, so a confirmed-real twin never keeps one.
+        reason = row.get("false_trigger_reason") if ft else None
         twins = self.find_twins(event_id)
         moved = []
         with self._connect() as conn:
             for tw in twins:
-                conn.execute("UPDATE events SET false_trigger=?, reviewed_real=? WHERE id=?",
-                             (ft, real, tw["id"]))
+                conn.execute(
+                    "UPDATE events SET false_trigger=?, reviewed_real=?, false_trigger_reason=? WHERE id=?",
+                    (ft, real, reason, tw["id"]))
                 moved.append(tw["id"])
         return moved
 
@@ -751,7 +757,7 @@ class SeismoDb:
                 )
             else:
                 cur = conn.execute(
-                    "UPDATE events SET false_trigger=0 WHERE id=?",
+                    "UPDATE events SET false_trigger=0, false_trigger_reason=NULL WHERE id=?",
                     (event_id,),
                 )
         return cur.rowcount > 0
@@ -845,7 +851,8 @@ class SeismoDb:
             return False
         has_ft = "false_trigger" in review
         has_real = "reviewed_real" in review
-        if not has_ft and not has_real:
+        has_reason = "false_trigger_reason" in review
+        if not has_ft and not has_real and not has_reason:
             # Nothing derived to update; just confirm the row exists.
             with self._connect() as conn:
                 row = conn.execute(
@@ -858,11 +865,19 @@ class SeismoDb:
             sets["false_trigger"] = 1 if review.get("false_trigger") else 0
         if has_real:
             sets["reviewed_real"] = 1 if review.get("reviewed_real") else 0
+        if has_reason:
+            reason = review.get("false_trigger_reason") or None
+            sets["false_trigger_reason"] = reason
+            if reason:                      # a reason is a subtype of FT → implies FT
+                sets["false_trigger"] = 1
         # mutual exclusivity: a true in one forces the other column to 0
         if sets.get("false_trigger") == 1:
             sets["reviewed_real"] = 0
         if sets.get("reviewed_real") == 1:
             sets["false_trigger"] = 0
+        # the reason is only meaningful while flagged FT — clear it if FT ends up 0
+        if sets.get("false_trigger") == 0:
+            sets["false_trigger_reason"] = None
         assign = ", ".join(f"{k}=?" for k in sets)
         params = list(sets.values()) + [event_id]
         with self._connect() as conn:

tests/test_ft_reason.py

@@ -0,0 +1,103 @@
+import sqlite3
+
+from sfm.database import SeismoDb
+from minimateplus.models import Event, Timestamp
+
+
+def _ev(db, key="0111aaaa", serial="BE1"):
+    ev = Event(index=0)
+    ev._waveform_key = bytes.fromhex(key)
+    ev.timestamp = Timestamp(raw=b"", flag=0x10, year=2026, unknown_byte=0,
+                             month=6, day=25, hour=8, minute=0, second=0)
+    ev.record_type = "Waveform"
+    db.insert_events([ev], serial=serial)
+    return [r for r in db.query_events(serial=serial) if r["waveform_key"] == key][0]["id"]
+
+
+def test_flag_offset_reason_implies_ft(tmp_path):
+    db = SeismoDb(tmp_path / "s.db")
+    eid = _ev(db)
+    db.update_event_review(eid, {"false_trigger_reason": "offset"})
+    row = db.get_event(eid)
+    assert row["false_trigger"] == 1          # a reason is a subtype of FT
+    assert row["false_trigger_reason"] == "offset"
+    assert row["reviewed_real"] == 0
+
+
+def test_plain_ft_leaves_reason_null(tmp_path):
+    # Reason is OPTIONAL — flagging FT without one records no reason.
+    db = SeismoDb(tmp_path / "s.db")
+    eid = _ev(db)
+    db.update_event_review(eid, {"false_trigger": True})
+    row = db.get_event(eid)
+    assert row["false_trigger"] == 1
+    assert row["false_trigger_reason"] is None
+
+
+def test_confirm_real_clears_reason(tmp_path):
+    db = SeismoDb(tmp_path / "s.db")
+    eid = _ev(db)
+    db.update_event_review(eid, {"false_trigger_reason": "offset"})
+    db.update_event_review(eid, {"reviewed_real": True})
+    row = db.get_event(eid)
+    assert row["reviewed_real"] == 1
+    assert row["false_trigger"] == 0
+    assert row["false_trigger_reason"] is None
+
+
+def test_clear_ft_clears_reason(tmp_path):
+    db = SeismoDb(tmp_path / "s.db")
+    eid = _ev(db)
+    db.update_event_review(eid, {"false_trigger_reason": "offset"})
+    db.update_event_review(eid, {"false_trigger": False})
+    row = db.get_event(eid)
+    assert row["false_trigger"] == 0
+    assert row["false_trigger_reason"] is None
+
+
+def test_set_false_trigger_false_clears_reason(tmp_path):
+    db = SeismoDb(tmp_path / "s.db")
+    eid = _ev(db)
+    db.update_event_review(eid, {"false_trigger_reason": "offset"})
+    assert db.set_false_trigger(eid, False) is True
+    row = db.get_event(eid)
+    assert row["false_trigger"] == 0
+    assert row["false_trigger_reason"] is None
+
+
+def test_reason_can_be_cleared_without_clearing_ft(tmp_path):
+    # Setting reason to None removes the reason but leaves the FT flag intact.
+    db = SeismoDb(tmp_path / "s.db")
+    eid = _ev(db)
+    db.update_event_review(eid, {"false_trigger_reason": "offset"})
+    db.update_event_review(eid, {"false_trigger_reason": None})
+    row = db.get_event(eid)
+    assert row["false_trigger"] == 1
+    assert row["false_trigger_reason"] is None
+
+
+def _ts(h, m, d=25):
+    return Timestamp(raw=b"", flag=0x10, year=2026, unknown_byte=0,
+                     month=2, day=d, hour=h, minute=m, second=0)
+
+
+def test_offset_reason_propagates_to_twin(tmp_path):
+    # Flag a waveform as offset → its histogram twin also becomes FT with reason=offset.
+    db = SeismoDb(tmp_path / "s.db")
+
+    def ins(key, ts, rt):
+        ev = Event(index=0); ev._waveform_key = bytes.fromhex(key); ev.timestamp = ts
+        db.insert_events([ev], serial="BE1")
+        rid = [r for r in db.query_events(serial="BE1") if r["waveform_key"] == key][0]["id"]
+        with sqlite3.connect(db.db_path) as c:
+            c.execute("UPDATE events SET peak_vector_sum=0.4763, record_type=? WHERE id=?", (rt, rid))
+        return rid
+
+    hist = ins("01110001", _ts(19, 31), "Histogram")   # interval start
+    wave = ins("01110002", _ts(20, 46), "Waveform")     # trigger inside the interval
+
+    db.update_event_review(wave, {"false_trigger_reason": "offset"})
+    db.propagate_review_to_twins(wave)
+    row = db.get_event(hist)
+    assert row["false_trigger"] == 1
+    assert row["false_trigger_reason"] == "offset"