Validate pushbutton internal connection pairs

[ShiboSoftwareDev]

Summary

• validate that multi-pin pushbuttons declare two complete internally connected pin pairs
• accept either component-level groups or source internal-connection records
• run the validation alongside existing netlist checks

Tests

• bun test tests/lib/check-push-button-internal-connections.test.ts
• bunx tsc --noEmit
• bun run build