fix(footprint): emit (layer B.Cu) for bottom-layer components

[gsdali]

Summary

When a circuit-json pcb_component has layer: "bottom", the KiCad
exporter currently emits (layer F.Cu) at the top of the resulting
(footprint ...) block. KiCad's pcbnew reads that header line to
populate the footprint's "Board Side" property, so every back-side
component shows up as "Board Side: Front" even though its pads,
silkscreen and paste are correctly placed on B.Cu / B.Paste / B.Mask
inside the same block.

Reproducible on a board with any layer="bottom" component:

<board width="20mm" height="14mm">
  <resistor name="R_BOTTOM" resistance="10k" footprint="0402"
    pcbX={5} pcbY={0} layer="bottom" />
</board>

Output before this PR — R_BOTTOM opens "Front" in pcbnew:

(footprint
  "tscircuit:resistor_0402"
  (layer F.Cu)        ← wrong
  ...
  (pad ... (layers B.Cu B.Paste B.Mask) ...)   ← pads are right

Fix

AddFootprintsStage._step builds the footprint with a hardcoded
layer: "F.Cu". Read component.layer from the circuit-json
pcb_component and map "bottom" → "B.Cu", everything else
("top", undefined, inner layers) keeps "F.Cu".

const footprintLayer = component.layer === "bottom" ? "B.Cu" : "F.Cu"
const footprint = new Footprint({
  libraryLink: `tscircuit:${footprintName}`,
  layer: footprintLayer,
  at: [...],
  uuid: ...,
})

Total diff: +9 / −1 lines in AddFootprintsStage.ts (most of which
is an explanatory comment), plus a new regression test.

Tests

tests/pcb/basics/basics17-bottom-layer-footprint.test.tsx builds
a board with one layer="top" and one layer="bottom" resistor,
extracts both (footprint ...) blocks from the rendered s-expression,
classifies each by whether any pad inside is on B.Cu, and asserts
the footprint header layer is F.Cu / B.Cu respectively.

The suite goes from 8 pass / 10 fail on main to 9 pass / 10 fail
on this branch — one more pass (the new test). The 10 pre-existing
failures are PNG-snapshot environmental diffs (KiCad CLI version)
unrelated to this fix; the basics01–basics05 / 06 / 09 / 10 / 14 / 15
PNGs differ on main as well.

Test plan

• New regression test passes locally
• Suite count check vs upstream/main: same fail count, +1 pass
• CI green
• Reviewer can open the test fixture in KiCad pcbnew and confirm
  the back-side resistor's "Properties → Board Side" reads "Back"

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
circuit-json-to-kicad	Ready	Preview, Comment	May 8, 2026 4:11am