refactor(seedspec): move seed spec parsing out of the campaign command

the campaign tool and the sweep tools that drive it have to read a seed specification the same way, or a sweep records an intent that differs from what ran. parseSeeds becomes seedspec.Parse with no behaviour change.

internal/seedspec/seeds_test.go

@@ -0,0 +1,49 @@
+package seedspec
+
+import (
+	"slices"
+	"strings"
+	"testing"
+)
+
+func TestParse_RangesAndLists(t *testing.T) {
+	cases := []struct {
+		specification string
+		want          []int64
+	}{
+		{"1-5", []int64{1, 2, 3, 4, 5}},
+		{"1,5,9", []int64{1, 5, 9}},
+		{"1-3,20,30-32", []int64{1, 2, 3, 20, 30, 31, 32}},
+		{" 7 , 8 ", []int64{7, 8}},
+		{"4-4", []int64{4}},
+	}
+	for _, testCase := range cases {
+		got, err := Parse(testCase.specification)
+		if err != nil {
+			t.Fatalf("%q: %v", testCase.specification, err)
+		}
+		if !slices.Equal(got, testCase.want) {
+			t.Errorf("%q: got %v, want %v", testCase.specification, got, testCase.want)
+		}
+	}
+}
+
+func TestParse_RejectsSeedZero(t *testing.T) {
+	for _, specification := range []string{"0", "1,0,2", "0-3"} {
+		_, err := Parse(specification)
+		if err == nil {
+			t.Fatalf("%q: expected rejection of seed 0", specification)
+		}
+		if !strings.Contains(err.Error(), "not reproducible") {
+			t.Errorf("%q: error should explain why seed 0 is rejected: %v", specification, err)
+		}
+	}
+}
+
+func TestParse_RejectsMalformed(t *testing.T) {
+	for _, specification := range []string{"", "   ", "abc", "1,,2", "5-1", "1-", "-5", "1-2-3", "1.5", "2,2"} {
+		if seeds, err := Parse(specification); err == nil {
+			t.Errorf("%q: expected error, got %v", specification, seeds)
+		}
+	}
+}