Add MoonWatch app, widget extension, shared module, and .gitignore

Adds the real MoonWatch implementation on top of the initial Xcode
template: new SwiftUI views (MoonDiskView, StarFieldView), the
MoonWatchWidget extension, and a Shared module containing the moon
phase calculator, shadow shape, and shared asset catalog. Also adds a
Swift/Xcode/macOS .gitignore and untracks per-user xcuserdata files.

Co-authored-by: Cursor <cursoragent@cursor.com>

MoonWatchTests/MoonWatchTests.swift

@@ -10,10 +10,42 @@ import Testing
 
 struct MoonWatchTests {
 
-    @Test func example() async throws {
-        // Write your test here and use APIs like `#expect(...)` to check expected conditions.
-        // Swift Testing Documentation
-        // https://developer.apple.com/documentation/testing
+    private let iso8601: ISO8601DateFormatter = {
+        let f = ISO8601DateFormatter()
+        f.formatOptions = [.withInternetDateTime]
+        return f
+    }()
+
+    @Test func julianDayUnixEpoch() {
+        let utc = try #require(iso8601.date(from: "1970-01-01T00:00:00Z"))
+        let jd = MoonPhaseCalculator.julianDay(for: utc)
+        #expect(abs(jd - 2_440_587.5) < 0.0001)
     }
 
+    /// Full moon near total lunar eclipse peak (UTC).
+    @Test func goldenFullMoonJanuary2019() {
+        let date = try #require(iso8601.date(from: "2019-01-21T05:16:00Z"))
+        let s = MoonPhaseCalculator.snapshot(for: date)
+        #expect(s.namedPhase == .fullMoon)
+        #expect(s.normalizedPhase > 0.5 && s.normalizedPhase < 0.52)
+        #expect(s.illuminatedFraction > 0.99)
+    }
+
+    /// New moon (UTC).
+    @Test func goldenNewMoonNovember2024() {
+        let date = try #require(iso8601.date(from: "2024-11-01T13:00:00Z"))
+        let s = MoonPhaseCalculator.snapshot(for: date)
+        #expect(s.namedPhase == .newMoon)
+        #expect(s.normalizedPhase >= 0 && s.normalizedPhase < 0.02)
+        #expect(s.illuminatedFraction < 0.02)
+    }
+
+    /// First quarter (UTC).
+    @Test func goldenFirstQuarterOctober2024() {
+        let date = try #require(iso8601.date(from: "2024-10-10T19:00:00Z"))
+        let s = MoonPhaseCalculator.snapshot(for: date)
+        #expect(s.namedPhase == .firstQuarter)
+        #expect(s.normalizedPhase > 0.25 && s.normalizedPhase < 0.28)
+        #expect(s.illuminatedFraction > 0.45 && s.illuminatedFraction < 0.60)
+    }
 }