chore: bump Go 1.18→1.26, fix typos, clean up tests

- Update go.mod to Go 1.26
- Fix help text typos ('menut' → 'menu', 'most-significant' → 'most-specific')
- Fix error comment typo ('ll' → 'all')
- Remove unnecessary fmt.Sprintf in test names
- Remove unused fmt imports from test files
- Add compile-time interface check for TimeWindow

msp/types.go

@@ -2,6 +2,9 @@ package msp
 
 import "time"
 
+// Compile-time interface check.
+var _ Period = TimeWindow{}
+
 type Period interface {
 	GetStartTime() time.Time
 	GetEndTime() time.Time