feat(quando-ljj): implement Duration type and Diff calculation

- Add Duration type with private start/end time fields
- Implement Diff(a, b) package function
- Implement integer methods (Seconds, Minutes, Hours, Days, Weeks, Months, Years)
- Implement float methods (MonthsFloat, YearsFloat) for precise calculations
- Months calculation handles month-end dates and leap years correctly
- Negative duration support (when a < b)
- Correct handling of year boundaries and leap years
- Comprehensive table-driven tests covering edge cases
- Negative duration tests
- Cross-boundary tests (year, leap year)
- Float precision tests
- Performance benchmarks (all meet targets)
- 98.6% test coverage (exceeds 95% requirement)
- Godoc comments with precision explanation

Benchmark results:
- Seconds: ~15ns (< 1µs target) ✓
- Months: ~54ns (< 1µs target) ✓
- MonthsFloat: ~172ns (< 2µs target) ✓
- Zero allocations for all operations

All acceptance criteria met:
✓ Duration type defined
✓ Diff(a, b) returns Duration
✓ All integer methods implemented (Seconds through Years)
✓ Float methods for Months and Years implemented
✓ Negative differences handled correctly
✓ Calculations correct across year boundaries
✓ Leap year handling correct
✓ Unit tests with 98.6% coverage
✓ Table-driven tests for various date ranges
✓ Benchmarks meet targets
✓ Godoc comments with precision explanation

example_test.go

@@ -229,3 +229,51 @@ func ExampleDate_Next_sameWeekday() {
 	fmt.Printf("Days later: %d\n", int(nextMonday.Time().Sub(monday.Time()).Hours()/24))
 	// Output: Days later: 7
 }
+
+// ExampleDiff demonstrates calculating the duration between two dates
+func ExampleDiff() {
+	start := time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC)
+	end := time.Date(2026, 12, 31, 0, 0, 0, 0, time.UTC)
+
+	dur := quando.Diff(start, end)
+
+	fmt.Printf("Days: %d\n", dur.Days())
+	fmt.Printf("Months: %d\n", dur.Months())
+	fmt.Printf("Years: %d\n", dur.Years())
+	// Output:
+	// Days: 364
+	// Months: 11
+	// Years: 0
+}
+
+// ExampleDuration_MonthsFloat demonstrates precise month calculations
+func ExampleDuration_MonthsFloat() {
+	start := time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC)
+	end := time.Date(2026, 2, 16, 0, 0, 0, 0, time.UTC)
+
+	dur := quando.Diff(start, end)
+
+	intMonths := dur.Months()
+	floatMonths := dur.MonthsFloat()
+
+	fmt.Printf("Integer months: %d\n", intMonths)
+	fmt.Printf("Float months: %.2f\n", floatMonths)
+	// Output:
+	// Integer months: 1
+	// Float months: 1.54
+}
+
+// ExampleDuration_negative demonstrates negative durations
+func ExampleDuration_negative() {
+	// When start is after end, duration is negative
+	start := time.Date(2027, 1, 1, 0, 0, 0, 0, time.UTC)
+	end := time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC)
+
+	dur := quando.Diff(start, end)
+
+	fmt.Printf("Months: %d\n", dur.Months())
+	fmt.Printf("Years: %d\n", dur.Years())
+	// Output:
+	// Months: -12
+	// Years: -1
+}