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>

project.yml

@@ -37,6 +37,7 @@ targets:
     deploymentTarget: "16.0"
     sources:
       - path: KisaniCal
+      - path: Shared
     dependencies:
       - target: KisaniCalWidgets
         embed: true
@@ -90,6 +91,7 @@ targets:
       - path: KisaniCalWidgets
       - path: KisaniCal/Managers/AppGroup.swift
       - path: KisaniCal/Managers/TaskActivityAttributes.swift
+      - path: Shared
     info:
       path: KisaniCalWidgets/Info.plist
       properties: