fix: change snap_to_grid default grid from 2.54mm to 1.27mm (50 mil)

Half of all valid KiCAD schematic pin positions are on the 50-mil
(1.27mm) grid but not the 100-mil (2.54mm) grid — e.g. 26.67mm = 21 ×
1.27mm. Snapping to 2.54mm displaced those coordinates by 1.27mm,
moving labels off their pins and increasing floating-label count.

KiCAD source confirms: DEFAULT_CONNECTION_GRID_MILS = 50 and the ERC
off-grid check uses exact integer modulo against this value, so any
displacement breaks connectivity unconditionally.

Also update the kicad-source absolute path in CLAUDE.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

python/kicad_interface.py

@@ -3121,7 +3121,7 @@ class KiCADInterface:
             if not schematic_path:
                 return {"success": False, "message": "schematicPath is required"}
 
-            grid_size = float(params.get("gridSize", 2.54))
+            grid_size = float(params.get("gridSize", 1.27))
             elements = params.get("elements")  # None → defaults inside snap_to_grid
 
             result = snap_to_grid(Path(schematic_path), grid_size=grid_size, elements=elements)