test: mark StreakLogicTests @MainActor to fix Swift 6 compile error (KC-72)

The CI runner was broken all session (registered but never actually
polled for jobs - separate infra bug, fixed alongside this). First
real CI run surfaced a genuine, pre-existing compile error unrelated
to tonight's work: StreakLogicTests called @MainActor-isolated
WorkoutViewModel/TaskViewModel static methods from a non-isolated
test class. RecurrenceTests.swift in the same target already uses
the correct @MainActor class annotation - StreakLogicTests was just
missing it.

Verified locally: full suite now passes, 78/78 tests, 0 failures,
on a real simulator (iPhone 17 Pro) - not just self-review.

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

KisaniCalTests/StreakLogicTests.swift

@@ -1,6 +1,7 @@
 import XCTest
 @testable import KisaniCal
 
+@MainActor
 final class StreakLogicTests: XCTestCase {
 
     private let cal = Calendar.current