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>

Shared/MoonPhase.swift

@@ -0,0 +1,50 @@
+//
+//  MoonPhase.swift
+//  Shared between MoonWatch and MoonWatchWidget.
+//
+
+import Foundation
+
+public enum MoonPhaseName: String, CaseIterable, Sendable {
+    case newMoon = "New Moon"
+    case waxingCrescent = "Waxing Crescent"
+    case firstQuarter = "First Quarter"
+    case waxingGibbous = "Waxing Gibbous"
+    case fullMoon = "Full Moon"
+    case waningGibbous = "Waning Gibbous"
+    case thirdQuarter = "Third Quarter"
+    case waningCrescent = "Waning Crescent"
+}
+
+public struct MoonPhaseSnapshot: Equatable, Sendable {
+    public let date: Date
+    /// Position in synodic month, 0 at new moon approaching 1 at the next new moon.
+    public let normalizedPhase: Double
+    /// Days since last new moon, in `[0, synodicMonth)`.
+    public let moonAgeDays: Double
+    public let namedPhase: MoonPhaseName
+    /// Fraction of the apparent disk illuminated, 0…1.
+    public let illuminatedFraction: Double
+    /// The next phase that will begin after the current one.
+    public let nextPhaseName: MoonPhaseName
+    /// Days until the next phase transition.
+    public let daysToNextPhase: Double
+
+    public init(
+        date: Date,
+        normalizedPhase: Double,
+        moonAgeDays: Double,
+        namedPhase: MoonPhaseName,
+        illuminatedFraction: Double,
+        nextPhaseName: MoonPhaseName,
+        daysToNextPhase: Double
+    ) {
+        self.date = date
+        self.normalizedPhase = normalizedPhase
+        self.moonAgeDays = moonAgeDays
+        self.namedPhase = namedPhase
+        self.illuminatedFraction = illuminatedFraction
+        self.nextPhaseName = nextPhaseName
+        self.daysToNextPhase = daysToNextPhase
+    }
+}