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>

fill_resistance/dialog.py

@@ -9,9 +9,9 @@ from dataclasses import dataclass
 
 from PySide6.QtCore import Qt
 from PySide6.QtWidgets import (QApplication, QCheckBox, QComboBox, QDialog,
-                               QDialogButtonBox, QFormLayout, QLabel,
-                               QLineEdit, QListWidget, QListWidgetItem,
-                               QVBoxLayout)
+                               QDialogButtonBox, QFormLayout, QHBoxLayout,
+                               QLabel, QLineEdit, QListWidget,
+                               QListWidgetItem, QVBoxLayout, QWidget)
 
 from . import config, skin
 
@@ -41,7 +41,8 @@ class Selection:
     adaptive: bool = True
     push_overlays: bool = False   # EXPERIMENTAL in-KiCad |J| overlays
     trim_enabled: bool = False    # EXPERIMENTAL low-current copper marking
-    trim_pct: float = 10.0        # threshold as % of the mean |J|
+    trim_mode: str = "pct"        # "pct" (% of the mean |J|) or "abs" (A/mm2)
+    trim_value: float = 10.0      # threshold in the unit trim_mode names
 
 
 class _Dialog(QDialog):
@@ -140,10 +141,23 @@ class _Dialog(QDialog):
         self.trim_check.setChecked(config.TRIM_ENABLED)
         form.addRow("Low-current copper:", self.trim_check)
 
-        self.trim_edit = QLineEdit(f"{config.TRIM_THRESHOLD_PCT:g}")
-        self.trim_edit.setEnabled(config.TRIM_ENABLED)
-        self.trim_check.toggled.connect(self.trim_edit.setEnabled)
-        form.addRow("Threshold [% of mean |J|]:", self.trim_edit)
+        self.trim_mode_box = QComboBox()
+        self.trim_mode_box.addItem("% of mean |J|", "pct")
+        self.trim_mode_box.addItem("A/mm²", "abs")
+        self.trim_mode_box.setCurrentIndex(1 if config.TRIM_MODE == "abs"
+                                           else 0)
+        self.trim_edit = QLineEdit(self._trim_default())
+        for w in (self.trim_edit, self.trim_mode_box):
+            w.setEnabled(config.TRIM_ENABLED)
+            self.trim_check.toggled.connect(w.setEnabled)
+        self.trim_mode_box.currentIndexChanged.connect(
+            self._trim_mode_changed)
+        trim_row = QWidget()
+        trim_lay = QHBoxLayout(trim_row)
+        trim_lay.setContentsMargins(0, 0, 0, 0)
+        trim_lay.addWidget(self.trim_edit, 1)
+        trim_lay.addWidget(self.trim_mode_box)
+        form.addRow("Threshold:", trim_row)
 
         buttons = QDialogButtonBox(QDialogButtonBox.Ok | QDialogButtonBox.Cancel)
         buttons.accepted.connect(self._try_accept)
@@ -172,6 +186,19 @@ class _Dialog(QDialog):
         self.net_box.currentTextChanged.connect(self._refresh)
         self._refresh()
 
+    def _trim_default(self, mode: str | None = None) -> str:
+        mode = mode or self.trim_mode_box.currentData()
+        return (f"{config.TRIM_THRESHOLD_A_MM2:g}" if mode == "abs"
+                else f"{config.TRIM_THRESHOLD_PCT:g}")
+
+    def _trim_mode_changed(self):
+        # swap in the new unit's default, but never clobber a number the
+        # user typed themselves
+        mode = self.trim_mode_box.currentData()
+        other = "pct" if mode == "abs" else "abs"
+        if self.trim_edit.text().strip() in ("", self._trim_default(other)):
+            self.trim_edit.setText(self._trim_default(mode))
+
     def _refresh(self):
         self.error_label.setVisible(False)
         net = self.net_box.currentText()
@@ -242,12 +269,15 @@ class _Dialog(QDialog):
             extra_cu = number(self.extracu_edit, "Extra Cu")
             if extra_cu < 0:
                 raise ValueError("Extra Cu must be ≥ 0 µm.")
-        trim_pct = config.TRIM_THRESHOLD_PCT
+        trim_mode = self.trim_mode_box.currentData()
+        trim_value = float(self._trim_default(trim_mode))
         if self.trim_check.isChecked():
-            trim_pct = number(self.trim_edit, "Trim threshold")
-            if not 0 < trim_pct < 100:
+            trim_value = number(self.trim_edit, "Trim threshold")
+            if trim_mode == "pct" and not 0 < trim_value < 100:
                 raise ValueError("Trim threshold must be between 0 and "
                                  "100 (% of the mean |J|).")
+            if trim_mode == "abs" and trim_value <= 0:
+                raise ValueError("Trim threshold must be > 0 A/mm².")
         cap_max_drill = config.CAP_MAX_DRILL_MM
         if self.capped_check.isChecked():
             cap_max_drill = number(self.cap_drill_edit, "Capped up to drill")
@@ -274,7 +304,7 @@ class _Dialog(QDialog):
                          adaptive=self.adaptive_check.isChecked(),
                          push_overlays=self.overlay_check.isChecked(),
                          trim_enabled=self.trim_check.isChecked(),
-                         trim_pct=trim_pct)
+                         trim_mode=trim_mode, trim_value=trim_value)
 
     def _try_accept(self) -> None:
         try: