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>

README.md

@@ -179,8 +179,13 @@ spelled out per step and in *Platform notes* below.
    `python tools/kicad_heatmap_overlay.py --net X --amps 10`.
 6. **Experimental — low-current copper marking** (dialog checkbox,
    default off): after the solve, the copper whose |J| is **below a
-   threshold** (dialog-settable, default 10 % of the mean |J| over all
-   solved copper) is outlined as **filled graphic polygons** on
+   threshold** is outlined as **filled graphic polygons** on user
+   layers. The threshold is dialog-settable in one of two units
+   (selector next to the field): **relative** — % of the mean |J| over
+   all solved copper (default, 10 %) — or **absolute** in **A/mm²**;
+   since |J| scales with the test current, the absolute variant is
+   meant to be used with the real operating current entered as test
+   current. Polygons land on
    `User.5`…`User.8` (`TRIM_LAYERS` in `fill_resistance/config.py`;
    enable them in Board Setup), copper layers mapped in stackup order,
    top first. Marked specks under `TRIM_MIN_AREA_MM2` (0.5 mm²) are