Fix negative zero in imperial feet-inches parser

When the user enters `-0' - 10"`, Python parses feet as -0.0.
The check `feet < 0` is False for negative zero, so the sign was
silently dropped. Use math.copysign to detect it correctly.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/tool/unit.py

@@ -199,8 +199,8 @@ class Unit(bonsai.core.tool.Unit):
                 if inches is None:
                     inches = 0
 
-                # If feet is negative, inches should also be negative (subtractive)
-                if feet < 0:
+                # If feet is negative (including -0), inches should also be negative (subtractive)
+                if math.copysign(1, feet) < 0:
                     inches = -inches
 
                 # Convert to meters