Trim threshold: absolute A/mm2 variant next to the relative one

The dialog threshold field gets a unit selector: % of mean |J|
(default, as before) or absolute A/mm2. The absolute variant applies
at the solved operating point - |J| scales with the test current, so
it is meant to be used with the real operating current entered as the
test current (config comment, README and dialog say so).

Switching units swaps in the other unit config default
(TRIM_THRESHOLD_A_MM2 = 1.0 for absolute) but never clobbers a number
the user typed. Validation stays per unit: 0..100 for %, > 0 for
A/mm2. pipeline.run takes trim_pct/trim_abs (at most one), compute()
mirrors that, and the JSON records threshold_mode + threshold_value
next to the resolved threshold_a_per_mm2.

Suite on dev 3.13 and the Python 3.9 mac-stack venv: 151 passed each;
dialog wiring exercised offscreen (defaults, unit swap, validation).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_trim.py

@@ -22,13 +22,31 @@ def test_low_current_mask_threshold():
     J = np.full((1, 4, 4), np.nan)
     J[0, :2, :] = 1.0                  # 8 cells carrying little
     J[0, 2, :2] = 100.0                # 2 hot cells; mean = 20.8
-    mask, thr = trim.low_current_mask(J, 10.0)
+    mask, thr = trim.low_current_mask(J, pct=10.0)
     assert thr == pytest.approx(2.08)
     assert mask[0, :2, :].all()
     assert not mask[0, 2, :2].any()
     assert not mask[0, 3, :].any()     # NaN = no copper, never marked
 
 
+def test_low_current_mask_absolute():
+    J = np.full((1, 3, 3), np.nan)
+    J[0, 0, :] = 0.5e6                 # 0.5 A/mm2 in A/m2
+    J[0, 1, :] = 2.0e6                 # 2 A/mm2
+    mask, thr = trim.low_current_mask(J, abs_a_mm2=1.0)
+    assert thr == pytest.approx(1.0e6)
+    assert mask[0, 0, :].all()
+    assert not mask[0, 1, :].any()
+
+
+def test_low_current_mask_needs_exactly_one_threshold():
+    J = np.ones((1, 2, 2))
+    with pytest.raises(ValueError):
+        trim.low_current_mask(J)
+    with pytest.raises(ValueError):
+        trim.low_current_mask(J, pct=10.0, abs_a_mm2=1.0)
+
+
 def test_mask_rectangle_polygon():
     m = np.zeros((20, 30), dtype=bool)
     m[5:15, 4:9] = True
@@ -80,7 +98,8 @@ def test_compute_and_json(tmp_path):
     J[0, :, :5] = 0.01                 # half of the top layer nearly dead
     J[1, :, :] = 10.0
     stack = _stack(names=["F.Cu", "B.Cu"], h_nm=1_000_000)
-    tr = trim.compute(SimpleNamespace(Jmag=J), stack, 10.0)
+    tr = trim.compute(SimpleNamespace(Jmag=J), stack, pct=10.0)
+    assert tr.mode == "pct" and tr.value == 10.0
     assert [lt.layer for lt in tr.layers] == ["F.Cu", "B.Cu"]
     assert tr.layers[0].polygons and not tr.layers[1].polygons
     assert tr.layers[0].marked_mm2 == pytest.approx(50.0)
@@ -94,6 +113,24 @@ def test_compute_and_json(tmp_path):
     ring = doc["layers"][0]["polygons"][0]["outline_mm"]
     assert all(0 <= x <= 5.5 and 0 <= y <= 10.0 for x, y in ring)
     assert "F.Cu" in trim.summary_line(tr)
+    assert "% of mean" in trim.summary_line(tr)
+
+
+def test_compute_absolute_mode(tmp_path):
+    J = np.full((1, 10, 10), np.nan)
+    J[0, :, :] = 10.0e6                # 10 A/mm2
+    J[0, :, :5] = 0.1e6                # 0.1 A/mm2: below 1 A/mm2
+    stack = _stack(names=["F.Cu"], h_nm=1_000_000)
+    tr = trim.compute(SimpleNamespace(Jmag=J), stack, abs_a_mm2=1.0)
+    assert tr.mode == "abs" and tr.value == 1.0
+    assert tr.threshold_a_mm2 == pytest.approx(1.0)
+    assert tr.layers[0].marked_mm2 == pytest.approx(50.0)
+    line = trim.summary_line(tr)
+    assert "|J| < 1 A/mm2" in line and "% of mean" not in line
+    doc = json.loads(trim.write_json(tmp_path, tr)
+                     .read_text(encoding="utf-8"))
+    assert doc["threshold_mode"] == "absolute"
+    assert doc["threshold_value"] == 1.0
 
 
 def test_trim_shape_proto():