Reject a duration with characters outside its parts (closes #215)

parseDuration fell back to an unanchored search for number-and-unit
pieces when time.ParseDuration failed, and skipped everything in
between. 1.0y became 0, so `snapshot create --prune --keep-newer-than
1.0y` deleted every snapshot of the backed-up names, the new one
included. 2.1w became one week and 1,5y five years. The fallback now
requires the whole input to be whole-number-and-unit parts with nothing
between them. A bare number is rejected before time.ParseDuration sees
it, since Go reads 0 and +0 as zero with no unit.

Judgement call: a space between number and unit (`30 days`) was
accepted and is now an error, matching Go's own units.

Model: opus-5-5

internal/vaultik/helpers_test.go

@@ -59,6 +59,7 @@ func TestParseDuration(t *testing.T) {
 		{"30s", 30 * time.Second, false},
 		{"6m", 6 * time.Minute, false},
 		{"1h", time.Hour, false},
+		{"1.5h", 90 * time.Minute, false},
 		// Extended calendar units.
 		{"30d", 30 * 24 * time.Hour, false},
 		{"3days", 3 * 24 * time.Hour, false},
@@ -73,11 +74,21 @@ func TestParseDuration(t *testing.T) {
 		{"1y6mo", 365*24*time.Hour + 180*24*time.Hour, false},
 		// Rejected inputs.
 		{"6", 0, true},   // bare number, no unit
+		{"0", 0, true},   // bare number; time.ParseDuration accepts it
+		{"+0", 0, true},  // bare number; time.ParseDuration accepts it
 		{"5x", 0, true},  // unknown unit
 		{"-5d", 0, true}, // negative, extended unit
 		{"-5h", 0, true}, // negative, Go unit
 		{"", 0, true},    // empty
 		{"garbage", 0, true},
+		// Characters outside the number-and-unit parts must not be
+		// skipped: 1.0y read as 0y would select every snapshot.
+		{"1.0y", 0, true},
+		{"2.1w", 0, true},
+		{"1,5y", 0, true},
+		{"30 days", 0, true},
+		{"30 days ago", 0, true},
+		{"x7d", 0, true},
 	}
 
 	for _, tt := range tests {