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/MoonShadowShape.swift

@@ -0,0 +1,62 @@
+//
+//  MoonShadowShape.swift
+//  Shared between MoonWatch and MoonWatchWidget.
+//
+
+import SwiftUI
+
+/// Geometric terminator: half-circle along the dark limb stitched to a
+/// half-ellipse whose horizontal axis = cos(2π·t)·R. Renders correctly at
+/// new (full dark), quarter (straight terminator), gibbous, and full
+/// (no shadow). Slight `oversizeFactor` lets a parent `Circle` clip keep
+/// the limb sharp while a small `.blur` softens the terminator.
+public struct MoonShadowShape: Shape {
+    public var phaseFraction: Double
+    public var oversizeFactor: CGFloat
+
+    public init(phaseFraction: Double, oversizeFactor: CGFloat = 1.04) {
+        self.phaseFraction = phaseFraction
+        self.oversizeFactor = oversizeFactor
+    }
+
+    public var animatableData: Double {
+        get { phaseFraction }
+        set { phaseFraction = newValue }
+    }
+
+    public func path(in rect: CGRect) -> Path {
+        let r = min(rect.width, rect.height) / 2
+        let outer = r * oversizeFactor
+        let cx = rect.midX
+        let cy = rect.midY
+        let raw = phaseFraction.truncatingRemainder(dividingBy: 1)
+        let u = raw < 0 ? raw + 1 : raw
+        let cosT = CGFloat(cos(2 * Double.pi * u))
+        let waning = u > 0.5
+        let bulge: CGFloat = waning ? -cosT * outer : cosT * outer
+        let kappa: CGFloat = 0.5522847498
+
+        var path = Path()
+        let top = CGPoint(x: cx, y: cy - outer)
+        path.move(to: top)
+        path.addArc(
+            center: CGPoint(x: cx, y: cy),
+            radius: outer,
+            startAngle: .degrees(270),
+            endAngle: .degrees(90),
+            clockwise: !waning
+        )
+        path.addCurve(
+            to: CGPoint(x: cx + bulge, y: cy),
+            control1: CGPoint(x: cx + bulge * kappa, y: cy + outer),
+            control2: CGPoint(x: cx + bulge,         y: cy + outer * kappa)
+        )
+        path.addCurve(
+            to: top,
+            control1: CGPoint(x: cx + bulge,         y: cy - outer * kappa),
+            control2: CGPoint(x: cx + bulge * kappa, y: cy - outer)
+        )
+        path.closeSubpath()
+        return path
+    }
+}