Add pcb import tests

[akhilles]

Note

Medium Risk
CI changes add a new private-fixture dependency/token and longer E2E execution paths, and the importer now overwrites outputs and proceeds past ERC/DRC errors, which could change user expectations and allow importing designs with known violations.

Overview
Extends the GitHub Actions E2E workflow to clone diodeinc/kicad-test-fixtures and run pcb import for every *.kicad_pro, then pcb build and pcb layout each imported board to validate the full import pipeline.

Refines pcb import behavior: --force now controls whether an existing output board directory is deleted/overwritten (otherwise import fails early), and ERC/DRC errors are downgraded from a blocking/interactive confirmation path to a warning that does not stop import (schematic/layout parity remains blocking).

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Pipeline while loop may silently swallow import failures

Medium Severity

The find ... | while read pattern runs the while loop in a subshell. While GitHub Actions uses set -eo pipefail, set -e behavior inside a while loop body running in a pipeline subshell is historically inconsistent across bash versions and can silently continue past a failing pcb import without aborting the step. Since this is a test meant to catch import failures, a silently passing step defeats its purpose. Using find -exec or a for loop over a glob (or adding || exit 1 after the pcb import call) would reliably propagate errors.