fix(magnetostatic): unbreak main's fmt and clippy gates

[ecto]

Why

Grid-free magnetostatic oracle + fix cancelling multilayer coils (#692, 34f31195) landed with two CI gates unsatisfied, so main itself is red and every open PR fails for reasons unrelated to its own diff — including #695, which only touches crates/vcad-ecad-export/src/gerber.rs.

Both were verified against origin/main directly, not inferred from a PR run.

1. cargo fmt --check — the first step in the Rust job, so nothing after it ran:

$ git show origin/main:crates/vcad-kernel-magnetostatic/tests/magnet_model.rs > /tmp/mm.rs
$ rustfmt --edition 2021 --check /tmp/mm.rs
-        assert!(b.x.abs().max(b.y.abs()) < 1e-6 * want.abs(), "off-axis leakage");
+        assert!(
+            b.x.abs().max(b.y.abs()) < 1e-6 * want.abs(),
+            "off-axis leakage"
+        );

2. cargo clippy -- -D warnings — the next gate, which the format failure was masking. Fixing only the formatting moved the red one step down the job, which is exactly what the first push of this PR showed:

error: manual implementation of `.is_multiple_of()`
  --> crates/vcad-kernel-magnetostatic/src/magnet.rs:50:12
   |
50 |         if k % 2 == 0 {
   |            ^^^^^^^^^^ help: replace with: `k.is_multiple_of(2)`
   = note: `-D clippy::manual-is-multiple-of` implied by `-D warnings`

What

• rustfmt --edition 2021 over crates/vcad-kernel-magnetostatic (9 files, 7 changed). No hand edits — every hunk is line-wrapping of a chained expression or a macro call.
• k % 2 == 0 → k.is_multiple_of(2) in Polarity::alternating. Identical behaviour on usize.

Verified locally, with CI's exact invocations

• cargo clippy --workspace --exclude vcad-desktop --features vcad-kernel-text/no-builtin-font -- -D warnings → clean. (Note CI does not pass --all-targets; with it, several unrelated crates have pre-existing test-only lints — out of scope here.)
• cargo test -p vcad-kernel-magnetostatic → 20 passed.
• rustfmt --check over the crate → clean.

Kept separate from #695 deliberately: an unrelated formatting-and-lint sweep does not belong in a Gerber export fix, and this unblocks every open PR, not just that one.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
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