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/config.py

@@ -102,10 +102,17 @@ TRIM_ENABLED = False            # dialog default: mark the copper below
                                 # cut list: copper carries little current
                                 # BECAUSE the rest carries it - removal
                                 # redistributes |J|, re-run after changes
-TRIM_THRESHOLD_PCT = 10.0       # threshold as % of the mean |J| over all
-                                # solved copper cells (mean, not max:
-                                # contact-corner spikes would dwarf a
-                                # max-relative threshold); dialog-settable
+TRIM_MODE = "pct"               # dialog default for the threshold unit:
+                                # "pct" (% of the mean |J|) or "abs"
+                                # (A/mm2)
+TRIM_THRESHOLD_PCT = 10.0       # relative threshold: % of the mean |J|
+                                # over all solved copper cells (mean, not
+                                # max: contact-corner spikes would dwarf
+                                # a max-relative threshold)
+TRIM_THRESHOLD_A_MM2 = 1.0      # absolute threshold [A/mm2]. |J| scales
+                                # with the test current, so this is only
+                                # meaningful with the real operating
+                                # current entered as test current
 TRIM_LAYERS = ("User.5", "User.6", "User.7", "User.8")
                                 # copper layers map here in stackup order
                                 # (top first); existing polygons on these