Initial import: KiCad zone resistance plugin

DC/AC resistance, power dissipation, and via/injection-area currents of
copper zone fills. KiCad 10 IPC-API plugin (kicad-python/kipy):
multi-layer via-coupled FDM solver, multi-part terminals via User.1/User.2
marker layers, pads as contacts, uniform-injection and equipotential
contact models, per-foil skin effect, optional solder/copper buildup on
mask openings. 54-case test suite incl. exact analytic references.

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

README.md

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+# Fill Resistance — KiCad 10 plugin
+
+Computes the **DC resistance of copper zone fills** between two contacts,
+**single- or multi-layer**: the chosen net's fills on the selected copper
+layers are solved as coupled finite-difference sheets linked by the net's
+**via and through-hole-pad barrels** (18 µm plating, configurable). Shows
+per-layer rasterized maps, potential, current density, and **power
+density**, reports **per-via currents** (via ampacity!) and total
+dissipation at a **selectable test current**. PNGs + a text summary are
+saved per run.
+
+Uses the KiCad **IPC API** (`kicad-python` / `kipy`), not the deprecated
+SWIG API. Requires KiCad **10.0.1+**.
+
+## Setup (one-time)
+
+1. **Enable the API server**: KiCad → Preferences → Plugins → check
+   *Enable KiCad API*.
+2. **Check the interpreter path** on the same page: should be
+   `C:\Program Files\KiCad\10.0\bin\pythonw.exe` (after a 9→10 upgrade it
+   can point at KiCad 9).
+3. **Deploy**:
+   ```powershell
+   powershell -ExecutionPolicy Bypass -File deploy.ps1        # junction (dev)
+   powershell -ExecutionPolicy Bypass -File deploy.ps1 -Mode Copy
+   ```
+4. **Restart KiCad**; first load builds the plugin venv (numpy, scipy,
+   matplotlib, PySide6 — takes minutes; the Ω button appears when done).
+   If stuck: Preferences → Plugins → *Recreate Plugin Environment*.
+
+## Usage
+
+1. Mark the current-injection terminals. Each terminal may have
+   **multiple parts** (all merged into one externally-bonded contact):
+   - **V+ rectangles on `User.1`**, **V− rectangles on `User.2`**
+     (marker layers, configurable via `ELECTRODE_POS_LAYER` /
+     `ELECTRODE_NEG_LAYER`), any number per side, axis-aligned;
+   - **pads** (real copper shape; through-hole pad contacts all layers,
+     SMD pad its own layer) — selected pads fill a side that has no
+     rectangles;
+   - legacy: exactly 2 selected contacts with no marker rectangles still
+     works; empty selection scans the whole board's marker layers.
+2. **Select the contacts**, click the **Fill Resistance** Ω button.
+3. In the **dialog**, pick the net (defaults to the selected pad's net),
+   check the **layers** to include, set each contact's layer scope
+   ("All selected layers" = bolted-lug/through contact), the **test
+   current**, and optionally a grid cell size. Multiple layers are coupled
+   through the net's via/pad barrels automatically.
+4. Read R / voltage drop / total power in the figure titles and status
+   bar. Outputs land in `<board dir>\fill_res_results\<timestamp>\`:
+   per-layer `1_raster_map` / `2_potential` / `3_current_density` /
+   `4_power_density` PNGs, `summary.txt` (incl. the busiest vias with
+   per-via current and dissipation, and the **current through each
+   injection area** — computed flux with the equipotential model,
+   prescribed area share with the uniform model), `geometry_dump.json`.
+
+## Model & limits
+
+- Sheet model per layer: R□ = ρ/t, ρ = 1.68e-8 Ωm (20 °C), t from the
+  board's physical stackup. Layer z-positions from the stackup drive the
+  barrel lengths.
+- Via/pad barrels: thin-wall annulus, R = ρ·L/(π·d·t_plating),
+  `VIA_PLATING_UM = 18` in `fill_resistance/config.py`. Vias are always
+  plated; capped vs uncapped does not change the layer-to-layer DC path
+  (the ≥5 µm cap sits over the hole mouth in parallel with the
+  annular-ring contact, not in series). A barrel passing an antipad
+  bridges the layers above/below with the full barrel length.
+- Tracks and pad copper (other than the selected contacts) are **not**
+  part of the conductor model — zone fills + barrels only.
+- **Solder buildup on mask openings** (dialog checkbox, **off by
+  default**; `INCLUDE_MASK_BUILDUP`): zones drawn on `F.Mask`/`B.Mask`
+  are treated as mask openings that collect `SOLDER_THICKNESS_UM`
+  (50 µm) of solder on the exposed pour, plus an optional user-defined
+  added copper thickness (dialog field, e.g. a soldered busbar/wire).
+  The sheet conductance there becomes t_Cu/ρ_Cu + t_solder/ρ_solder +
+  t_extra/ρ_Cu (SAC305 ρ = 1.32e-7 Ωm: 50 µm solder ≈ 6.4 µm copper);
+  interface faces use harmonic-mean conductances. Buildup areas render
+  tin-gray on the raster map; |J| in them is referenced to the
+  conductance-equivalent copper thickness.
+- **Contact models** (dialog / `CONTACT_MODEL`): default **uniform
+  injection** — a conductor pressed on top feeds the current orthogonally
+  with uniform surface density, so |J| ramps across the contact area
+  (R = ΔV̄/I from area-averaged terminal potentials); or
+  **equipotential** — ideal bonded lug (Dirichlet). The two bracket a
+  real contact: R_equipotential ≤ R_real ≤ R_uniform.
+- Fields are reported at the dialog's test current; power scales with I².
+- **Skin effect (f > 0)**: per-layer effective sheet resistance from the
+  exact 1D foil-diffusion solution `Zs = τρ·coth(τt)`, `τ = (1+j)/δ`
+  (`SKIN_SIDES = 1` in config: plane facing a return plane; `2` =
+  isolated foil), and the analogous correction for the 18 µm barrel wall.
+  Enter one frequency per run (e.g. a switching harmonic, with its RMS
+  amplitude as the test current) — suffixes `k`/`M` accepted.
+  **Caveat:** only through-thickness crowding is modeled. Lateral
+  (proximity-effect) redistribution needs a magneto-quasistatic solver
+  and is not captured — since the resistance-driven distribution is the
+  minimum-dissipation one, AC results are a rigorous **lower bound**.
+  Rule of thumb for 70 µm foil: skin is negligible below ~300 kHz
+  (δ = 173 µm at 142 kHz), ~+11 % at 1 MHz.
+- 5-point FDM per layer on an auto-sized shared grid (~500 k cells total
+  across layers by default). Direct sparse solve up to 700 k unknowns,
+  Jacobi-CG above. Discretization error typically ≲ 2 % at defaults —
+  halve the cell size and compare to judge convergence.
+
+## Offline / development
+
+Every run writes `geometry_dump.json`; re-solve without KiCad:
+
+```powershell
+.venv\Scripts\python.exe -m fill_resistance.standalone dump.json `
+    [--current 40] [--cell-um 50] [--layers F.Cu,In1.Cu] [--no-show] `
+    [--out DIR] [--force-iterative]
+```
+
+Dev environment, tests, headless extraction:
+
+```powershell
+uv venv --python 3.11 .venv
+uv pip install --python .venv\Scripts\python.exe kicad-python numpy scipy matplotlib pytest
+.venv\Scripts\python.exe -m pytest tests -q          # incl. exact analytic cases
+.venv\Scripts\python.exe smoke\smoke_probe.py        # IPC API probe vs live KiCad
+.venv\Scripts\python.exe -m fill_resistance.board_io dump.json [NET]  # extract only
+```
+
+## Troubleshooting
+
+- **No toolbar button**: venv still building (wait), or build failed →
+  *Recreate Plugin Environment*; check the interpreter path (setup 2).
+- **"Could not connect to KiCad's IPC API"**: API server not enabled, or
+  KiCad not running (no headless mode in KiCad 10).
+- **"KiCad is busy"**: a modal dialog is open in KiCad — close it, rerun.
+- **Windows don't appear**: they may open behind KiCad (raised
+  best-effort); PNGs are always saved regardless.
+- **Result seems too low/high**: remember the model is fills + barrels
+  only, with ideal contacts; measure electrode-to-electrode.