test: strengthen uppercase canonical test + add _detect_trend boundary cases

- Assert captured.err == "" in test_subtext_read_uppercase_canonical_coerced to verify no warning is emitted when .lower() hits the canonical directly
- Add test_detect_trend_rising_exactly_3: confirms len==3 is the minimum valid trend
- Add test_detect_trend_run_length_2_returns_none: confirms a tail run of 2 is below threshold

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_schemas.py

@@ -653,8 +653,8 @@ def test_subtext_read_unknown_falls_back_to_language_drift(capsys):
 def test_subtext_read_uppercase_canonical_coerced(capsys):
     s = _str(signal_type="Accounting_Quality")
     assert s.signal_type == "accounting_quality"
-    # The .lower() fix means the canonical is recognised directly — no stderr expected
-    capsys.readouterr()  # consume any output
+    captured = capsys.readouterr()
+    assert captured.err == ""  # .lower() hits canonical directly — no coercion warning
 
 
 def test_subtext_read_evidence_is_sourced_fact():
@@ -703,6 +703,16 @@ def test_detect_trend_too_short_returns_none():
     assert _detect_trend([1, 3]) is None
 
 
+def test_detect_trend_rising_exactly_3():
+    """Exactly 3 strictly increasing values — the minimum for a valid trend."""
+    assert _detect_trend([1, 2, 3]) == "rising 3 quarters"
+
+
+def test_detect_trend_run_length_2_returns_none():
+    """Tail run of exactly 2 — below the 3-quarter minimum threshold."""
+    assert _detect_trend([1, 2, 1, 2]) is None
+
+
 def test_detect_trend_empty_returns_none():
     assert _detect_trend([]) is None