coverage: easy-wins batch across cli, system, paths, vmdns, toolingplan

Pure-Go tests for formatters, layout resolution, and validators — no
fixtures, no external processes. Targets previously-zero functions the
triage scan flagged as low-hanging fruit.

  cli          55% -> 65%
  paths        64% -> 91%
  system       65% -> 75%
  vmdns        72% -> 86%
  toolingplan  73% -> 78%

Total 52.6% -> 54.0%.

internal/toolingplan/rust_test.go

@@ -0,0 +1,23 @@
+package toolingplan
+
+import "testing"
+
+func TestFirstMeaningfulLine(t *testing.T) {
+	cases := []struct {
+		in, want string
+	}{
+		{"", ""},
+		{"\n\n\n", ""},
+		{"   \n  \n", ""},
+		{"# just a comment\n# another\n", ""},
+		{"1.75.0\n", "1.75.0"},
+		{"  1.75.0  ", "1.75.0"},
+		{"# pinned toolchain\n1.75.0\nmore junk\n", "1.75.0"},
+		{"\n\n  stable-x86_64-unknown-linux-gnu  \n", "stable-x86_64-unknown-linux-gnu"},
+	}
+	for _, tc := range cases {
+		if got := firstMeaningfulLine(tc.in); got != tc.want {
+			t.Errorf("firstMeaningfulLine(%q) = %q, want %q", tc.in, got, tc.want)
+		}
+	}
+}