Add build gate for unrouted PCB nets

[seveibar]

Summary

• make tsci build exit with code 1 when traced PCB nets are not fully routed
• compare logical source-trace nets with physical PCB connectivity and report disconnected groups
• account for internal component connections and PCB layers when evaluating connectivity
• keep unrouted boards fatal even with --ignore-errors, while preserving existing behavior for other DRC diagnostics

Test plan

• bun test tests/shared/analyze-pcb-routing-completeness.test.ts tests/cli/build/build-fail-on-unrouted.test.ts
• bun test tests/cli/build/build-fail-on-unrouted.test.ts tests/shared/analyze-pcb-routing-completeness.test.ts tests/cli/build/build-with-drc-error.test.ts tests/cli/build/build-with-ignore-error.test.ts tests/cli/build/build-ignore-placement-drc-routes.test.ts tests/cli/build/build-concurrency03.test.ts
• bunx tsc --noEmit
• bun run format:check
• bun run build

The complete bun test run could not finish in this local environment because Sharp's native binary and Playwright Chromium are not installed; the run also encountered existing init/token-test timeouts. The focused and adjacent build suites above pass.