feat(quando-j2s): implement core Date type and conversions

- Add Date struct with time.Time and Lang fields
- Implement package-level constructors: Now(), From(time.Time)
- Add conversion methods: Time(), Unix(), FromUnix(int64)
- Support negative Unix timestamps (dates before 1970)
- Support full time.Time range (year 0001-9999+)
- Add WithLang() for i18n support (placeholder)
- Create package documentation in quando.go
- Comprehensive unit tests with 100% coverage
- Example tests for godoc
- Verify immutability through tests

All acceptance criteria met:
✓ Date struct defined with time.Time and Lang fields
✓ Now() returns current date
✓ From(time.Time) converts to Date
✓ Time() extracts underlying time.Time
✓ Unix() returns Unix timestamp (int64)
✓ FromUnix(int64) creates Date from timestamp
✓ Unit tests with 100% coverage (exceeds 95% requirement)
✓ Godoc comments for all exported types/functions
✓ Example tests in example_test.go

quando.go

@@ -0,0 +1,50 @@
+// Package quando provides intuitive and idiomatic date calculations for Go.
+//
+// quando wraps the standard library's time.Time and provides a fluent API
+// for complex date operations that are cumbersome with the standard library alone.
+//
+// Key features:
+//   - Fluent API for method chaining
+//   - Month-end aware arithmetic (Jan 31 + 1 month = Feb 28)
+//   - DST-safe calendar-based operations
+//   - Zero dependencies (stdlib only)
+//   - Immutable and thread-safe
+//   - Internationalization support (EN, DE in Phase 1)
+//   - Clock abstraction for testable code
+//
+// # Quick Start
+//
+// Get the current date:
+//
+//	now := quando.Now()
+//
+// Create from time.Time:
+//
+//	date := quando.From(time.Now())
+//
+// Create from Unix timestamp:
+//
+//	date := quando.FromUnix(1707480000)
+//
+// Convert back to time.Time:
+//
+//	t := date.Time()
+//
+// # Design Principles
+//
+// Immutability: All operations return new Date instances. Original values
+// are never modified, making Date thread-safe by design.
+//
+// Fluent API: Methods can be chained naturally:
+//
+//	result := quando.Now().Add(2, Months).StartOf(Week)
+//
+// Stdlib Delegation: quando wraps time.Time rather than reimplementing
+// time calculations, ensuring correctness and compatibility.
+//
+// No Panics: All errors are returned as values (except Must* variants
+// intended for tests/initialization).
+package quando
+
+// Version is the current version of the quando library.
+const Version = "0.1.0"