fix(units): declare ~P as dspeed's unit format default

dspeed assumes short-form unit names (e.g. 'ns' rather than 'nanosecond')
in output attrs and elsewhere. Today this works because lgdo.units sets
formatter.default_format = "~P" on the application registry when lh5
imports it. Make the dependency explicit so dspeed does not silently
break if that transitive import side effect goes away or if a consumer
uses dspeed.units without triggering lh5.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: gipert

src/dspeed/units.py

@@ -1,3 +1,6 @@
 import pint
 
 unit_registry = pint.get_application_registry()
+# match the short-form default used across the legend-exp ecosystem
+# (lgdo.units does the same on import)
+unit_registry.formatter.default_format = "~P"