chore(outputs): regenerate STEP and PCBWay fabrication package

* firstpcb.step regenerated from current PCB (DRC-clean board with
  four M2 mounting holes). Two component 3D models (R_0805 and
  JST_XH_B4B-XH-A) fail to decompress under the Nix-packaged KiCad;
  affected components are missing 3D shapes but board body and other
  models are present.
* pcbway_export/ contains gerbers (RS-274X Protel, 4.6 mm), separate
  PTH/NPTH Excellon drill files, drill maps, gbrjob, and a README
  with the stackup/spec.
* firstpcb_pcbway.zip is the upload-ready archive for PCBWay.
* .gitignore updated to drop KiCad autosaves and *.pre-drc-fix locals.

Constraint: PCBWay accepts gerbers with Protel extensions and Excellon
  drills, separate PTH/NPTH preferred
Directive: regenerate this package whenever the PCB changes -- the zip
  is a derived artifact; the source of truth is firstpcb.kicad_pcb

.gitignore

@@ -4,6 +4,12 @@
 # KiCad regenerated footprint info cache
 fp-info-cache
 
+# KiCad autosaved backups
+firstpcb-backups/
+
+# Local pre-edit backups
+*.pre-drc-fix
+
 # Python bytecode cache
 __pycache__/
 *.pyc