fix: invert suppress_threshold semantics to similarity_threshold in FalsePositiveSuppressor

Was suppressing when novelty_score < 0.85 (i.e. similarity > 0.15), which
would suppress nearly every hypothesis once embeddings are active.

Now suppresses when max_sim >= similarity_threshold (0.85), meaning only
hypotheses that are 85%+ similar to a resolved incident are suppressed.

Also renames suppress_threshold → similarity_threshold for clarity and
adds a borderline boundary test (0.85 suppressed, 0.84 not suppressed).

Closes: #29

app/services/diagnose/suppressor.py

@@ -113,12 +113,14 @@ class FalsePositiveSuppressor:
         self,
         model_id: str = "",
         device: str = "cpu",
-        suppress_threshold: float = 0.85,
+        similarity_threshold: float = 0.85,
     ) -> None:
         self._model_id = model_id
         self._device = device
         # _device stored for future use when get_embedder() supports device selection
-        self._suppress_threshold = suppress_threshold
+        # Suppress when cosine similarity to a known resolved incident >= threshold.
+        # A threshold of 0.85 means "suppress if 85%+ similar to something already resolved."
+        self._similarity_threshold = similarity_threshold
 
     def suppress(
         self,
@@ -199,7 +201,7 @@ class FalsePositiveSuppressor:
             )
 
         novelty_score = 1.0 - max_sim
-        suppress = bool(novelty_score < self._suppress_threshold)
+        suppress = bool(max_sim >= self._similarity_threshold)
         suppression_reason = (
             f"Similar to resolved incident (similarity {max_sim:.2f})"
             if suppress

tests/test_diagnose_suppressor.py

@@ -139,7 +139,7 @@ def test_high_similarity_suppresses_hypothesis(tmp_path):
         [("OOM killer", "Memory pressure caused OOM kill")],
         tmp_path / "turnstone.db",
     )
-    suppressor = FalsePositiveSuppressor(model_id="test-model", suppress_threshold=0.85)
+    suppressor = FalsePositiveSuppressor(model_id="test-model", similarity_threshold=0.85)
 
     with patch.object(suppressor, "_load_embedder", return_value=mock_embedder):
         results = suppressor.suppress([_make_hypothesis()], db_path)
@@ -171,7 +171,7 @@ def test_low_similarity_does_not_suppress(tmp_path):
         [("Disk I/O", "Storage saturation caused latency")],
         tmp_path / "turnstone.db",
     )
-    suppressor = FalsePositiveSuppressor(model_id="test-model", suppress_threshold=0.85)
+    suppressor = FalsePositiveSuppressor(model_id="test-model", similarity_threshold=0.85)
 
     with patch.object(suppressor, "_load_embedder", return_value=mock_embedder):
         results = suppressor.suppress([_make_hypothesis()], db_path)
@@ -184,6 +184,51 @@ def test_low_similarity_does_not_suppress(tmp_path):
     assert result.novelty_score == pytest.approx(1.0, abs=0.01)
 
 
+# ---------------------------------------------------------------------------
+# Test 3b: Borderline similarity — exactly at threshold vs. just below
+# ---------------------------------------------------------------------------
+
+def test_similarity_threshold_boundary(tmp_path):
+    """similarity == threshold is suppressed; similarity just below threshold is not.
+
+    This test locks down the boundary semantics: suppress when max_sim >= threshold,
+    not when novelty_score < threshold (the inverted form that was the original bug).
+    With threshold=0.85:
+      - similarity=0.85 → suppressed (at boundary, inclusive)
+      - similarity=0.84 → NOT suppressed (just below)
+    """
+    db_path = _make_db_with_incidents(
+        [("Disk I/O", "Storage saturation caused latency")],
+        tmp_path / "turnstone.db",
+    )
+
+    # Corpus unit vector along first axis
+    corpus_vec = [1.0] + [0.0] * 383
+
+    for sim_value, expected_suppress in [(0.85, True), (0.84, False)]:
+        # Build a hypothesis embedding whose cosine similarity to corpus_vec ≈ sim_value.
+        # query = [sim, sqrt(1 - sim^2), 0, ...] → cosine sim = sim exactly.
+        import math
+        hyp_vec = [sim_value, math.sqrt(max(0.0, 1.0 - sim_value ** 2))] + [0.0] * 382
+
+        mock_embedder = _make_mock_embedder(
+            embed_return=hyp_vec,
+            embed_batch_return=[corpus_vec],
+        )
+
+        suppressor = FalsePositiveSuppressor(model_id="test-model", similarity_threshold=0.85)
+
+        with patch.object(suppressor, "_load_embedder", return_value=mock_embedder):
+            results = suppressor.suppress([_make_hypothesis()], db_path)
+
+        assert len(results) == 1
+        result = results[0]
+        assert result.suppress is expected_suppress, (
+            f"similarity={sim_value:.2f}: expected suppress={expected_suppress}, "
+            f"got suppress={result.suppress} (similarity_to_known={result.similarity_to_known:.4f})"
+        )
+
+
 # ---------------------------------------------------------------------------
 # Test 4: Empty hypotheses list returns []
 # ---------------------------------------------------------------------------
@@ -259,7 +304,7 @@ def test_ranking_by_novelty_times_confidence(tmp_path):
         [("OOM", "Memory exhaustion")],
         tmp_path / "turnstone.db",
     )
-    suppressor = FalsePositiveSuppressor(model_id="test-model", suppress_threshold=0.85)
+    suppressor = FalsePositiveSuppressor(model_id="test-model", similarity_threshold=0.85)
 
     with patch.object(suppressor, "_load_embedder", return_value=mock_embedder):
         results = suppressor.suppress([h_a, h_b, h_c], db_path)
@@ -362,7 +407,7 @@ def test_corpus_cache_invalidated_on_corpus_change(tmp_path):
     mock_embedder.embed.return_value = single_m
     mock_embedder.embed_batch.side_effect = [[batch_m1], [batch_m2]]
 
-    suppressor = FalsePositiveSuppressor(model_id="test-model", suppress_threshold=0.85)
+    suppressor = FalsePositiveSuppressor(model_id="test-model", similarity_threshold=0.85)
 
     with patch.object(suppressor, "_load_embedder", return_value=mock_embedder):
         # First call — populates cache