Add Freerouting autoroute script

- autoroute.sh: Downloads and runs Freerouting in batch mode
- Updated README with complete PCB layout workflow
- Script handles DSN→SES conversion automatically

Usage: Export DSN from KiCad, run ./autoroute.sh, import SES

README.md

@@ -62,6 +62,25 @@ SN-L00/
 
 Full BOM with LCSC part numbers in [hardware/BOM.md](hardware/BOM.md).
 
+### PCB Layout (KiCad 8)
+
+1. Open `hardware/kicad/SN-L00.kicad_pro`
+2. Update PCB from schematic: **Tools → Update PCB from Schematic** (F8)
+3. Place components using the script:
+   ```python
+   # In KiCad Scripting Console (Tools → Scripting Console)
+   exec(open('scripts/place_components.py').read())
+   ```
+4. Autoroute using Freerouting:
+   ```bash
+   # Export DSN from KiCad: File → Export → Specctra DSN
+   ./hardware/kicad/scripts/autoroute.sh
+   # Import SES in KiCad: File → Import → Specctra Session
+   ```
+5. Fill zones: Press **B**
+6. Run DRC: **Inspect → Design Rules Checker**
+7. Export Gerbers: **File → Fabrication Outputs → Gerbers**
+
 ### Manufacturing
 
 - PCB: 2-layer, 28×100mm