fix(fundamentals): A4a review — stricter null semantics in derivation

1. net_debt requires BOTH cash and total_debt; a missing side is null, not
   treated as zero (which would be a partial, misleading value).
2. net_debt_to_ebitda is null when TTM EBITDA <= 0 — a negative denominator
   would otherwise rank a distressed issuer as favorably low-leverage.
3. The quarter tape is the CONSECUTIVE run ending at the latest period (stops at
   a gap), so trend text never compares non-adjacent quarters as if consecutive.
4. YoY growth is null when the prior-year TTM is <= 0 (e.g. loss->profit), which
   is not a meaningful percentage.

Also corrected the plan's net-debt formula to total debt − (cash + ST) matching
the positive-means-net-debt implementation. +4 tests. 10 passed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_fundamentals_derivation.py

@@ -123,6 +123,43 @@ def test_missing_period_yields_null_never_partial():
     assert d.ttm_diluted_eps is None
 
 
+def test_net_debt_requires_both_components():
+    rows = _two_years()
+    for r in rows:  # drop debt on the latest year -> can't form net debt
+        if r.fiscal_year == 2026:
+            r.total_debt = None
+    d = fd.derive(rows)
+    assert d.metrics["net_debt"].value is None
+    assert d.metrics["net_debt_to_ebitda"].value is None  # net debt null -> leverage null
+
+
+def test_leverage_null_when_ebitda_nonpositive():
+    rows = _two_years()
+    for r in rows:  # negative operating income -> TTM EBITDA <= 0
+        r.operating_income = -abs(r.revenue)
+        r.depreciation_amortization = 1
+    d = fd.derive(rows)
+    assert d.metrics["net_debt"].value == pytest.approx(100.0)  # net debt still valid
+    assert d.metrics["net_debt_to_ebitda"].value is None        # but leverage nulled
+
+
+def test_tape_stops_at_a_gap():
+    rows = [r for r in _two_years() if not (r.fiscal_year == 2026 and r.fiscal_period == "Q1")]
+    d = fd.derive(rows)
+    hist = d.metrics["operating_margin"].history
+    # consecutive suffix ending at FY2026: Q2, Q3, FY (not compressed across the Q1 gap)
+    assert [p.period_end for p in hist] == [date(2026, 3, 31), date(2026, 6, 30), date(2026, 9, 30)]
+
+
+def test_yoy_growth_null_when_prior_nonpositive():
+    rows = _two_years()
+    for r in rows:  # prior-year TTM EPS becomes negative
+        if r.fiscal_year == 2025:
+            r.diluted_eps = -abs(r.diluted_eps)
+    d = fd.derive(rows)
+    assert d.metrics["eps_growth_yoy"].value is None  # loss->profit is not a %
+
+
 def test_amendment_selection_newest_accepted_wins():
     rows = _two_years()
     # an amendment to FY2026 FY restates revenue YTD higher, accepted later