sud_0010: reset grain_mass to 0 at mash-in step

The "Einmaischen einfüllen" step waits for the user to add grain, so
grain_mass starts at 0 here and is added by the following steps.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JLR2133M2PyjiYq584i2G7

sude/sud_0010.json

@@ -53,6 +53,9 @@
 			}
 		},
 		{
+			"pot": {
+				"grain_mass": 0
+			},
 			"descr": "Einmaischen einfüllen",
 			"user_message": "Bitte Malz einfüllen und bestätigen",
 			"user_wait_for_continue": true,