recurrence: consolidate 3 duplicate implementations into one shared, tested engine (KC-69)

Before adding "every N weeks/months" support, found the recurrence
window math was independently hand-written in three places (main app's
TaskItem.isOccurrence, and two separate widget extensions' countdown
progress calculations) with no shared code to keep them in sync.

Extracted the math into Shared/RecurrenceRule.swift (pure Foundation,
no framework deps) compiled into both the app and widget extension
targets via a new project.yml Shared/ source path, and pointed all
three call sites at it. Verified every case is identical to prior
behavior at interval==1 by compiling and running a standalone test
driver directly against the real file with swiftc - 38/38 passing,
including Jan-31-monthly and Feb-29-leap-year-yearly edge cases.

Groundwork only - no recurrenceInterval field on TaskItem yet, the
actual interval feature comes next on top of this.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCalWidgets/EventCountdownWidget.swift

@@ -341,30 +341,11 @@ struct EventProvider: AppIntentTimelineProvider {
 
     /// Mode B window for a recurring event: the occurrence span containing "now"
     /// (previous occurrence → next occurrence), derived from the rule label.
+    /// The actual math lives in RecurrenceRule (Shared/) — shared with the main
+    /// app's occurrence matching and the other countdown widget's copy of this
+    /// same calculation, so none of the three can independently drift.
     private static func recurrenceSpan(label: String, due: Date) -> (prev: Date, next: Date)? {
-        let cal = Calendar.current
-        let now = Date()
-        let comp: Calendar.Component
-        let step: Int
-        switch label {
-        case "Daily", "Every Weekday": comp = .day;   step = 1
-        case "Weekly":                 comp = .day;   step = 7
-        case "Monthly":                comp = .month; step = 1
-        case "Yearly":                 comp = .year;  step = 1
-        default: return nil
-        }
-        var next = due
-        var guardRail = 0
-        if next > now {
-            // Walk back until the previous occurrence is in the past.
-            while let prev = cal.date(byAdding: comp, value: -step, to: next),
-                  prev > now, guardRail < 4000 { next = prev; guardRail += 1 }
-        } else {
-            while next <= now, let n = cal.date(byAdding: comp, value: step, to: next),
-                  guardRail < 4000 { next = n; guardRail += 1 }
-        }
-        guard let prev = cal.date(byAdding: comp, value: -step, to: next) else { return nil }
-        return (prev, next)
+        RecurrenceRule.span(label: label, due: due, now: Date())
     }
 }