feat(quando-4bh): implement Unit type and constants

- Define Unit type as int for type-safe time unit constants
- Implement all 8 unit constants (Seconds through Years) using iota
- Add String() method for debugging and error messages
- Comprehensive unit tests verifying constants, ordering, and comparability
- Type safety tests demonstrating compile-time safety
- Example tests showing usage patterns
- 100% test coverage (exceeds 95% requirement)

All acceptance criteria met:
✓ Unit type defined as int
✓ All 8 unit constants defined (Seconds through Years)
✓ Units use iota for ordering
✓ Godoc comments for Unit type and constants
✓ Unit tests verifying constants

unit.go

@@ -0,0 +1,57 @@
+package quando
+
+// Unit represents a time unit for arithmetic operations like Add and Sub,
+// and for snap operations like StartOf and EndOf.
+//
+// Use the predefined constants (Seconds, Minutes, Hours, Days, Weeks, Months,
+// Quarters, Years) rather than creating Unit values directly.
+type Unit int
+
+// Time unit constants for use with arithmetic and snap operations.
+// These constants are ordered from smallest to largest unit, except for Quarters
+// which is a special alias for 3 months.
+const (
+	// Seconds represents the seconds time unit.
+	Seconds Unit = iota
+	// Minutes represents the minutes time unit.
+	Minutes
+	// Hours represents the hours time unit.
+	Hours
+	// Days represents the days time unit.
+	// When used with Add, this means calendar days (not 24-hour periods).
+	Days
+	// Weeks represents the weeks time unit (7 days).
+	Weeks
+	// Months represents the months time unit.
+	// Adding months handles month-end overflow (e.g., Jan 31 + 1 month = Feb 28).
+	Months
+	// Quarters represents the quarters time unit (3 months).
+	Quarters
+	// Years represents the years time unit.
+	Years
+)
+
+// String returns the string representation of the Unit.
+// This is primarily useful for debugging and error messages.
+func (u Unit) String() string {
+	switch u {
+	case Seconds:
+		return "seconds"
+	case Minutes:
+		return "minutes"
+	case Hours:
+		return "hours"
+	case Days:
+		return "days"
+	case Weeks:
+		return "weeks"
+	case Months:
+		return "months"
+	case Quarters:
+		return "quarters"
+	case Years:
+		return "years"
+	default:
+		return "unknown"
+	}
+}