refactor(tests): TDD rewrite of all unit tests with spec documentation

Complete rewrite of unit test suite using TDD methodology:

Planning Engine Tests:
- GameDAGRouterTests: Beam search, anchor games, transitions
- ItineraryBuilderTests: Stop connection, validators, EV enrichment
- RouteFiltersTests: Region, time window, scoring filters
- ScenarioA/B/C/D PlannerTests: All planning scenarios
- TravelEstimatorTests: Distance, duration, travel days
- TripPlanningEngineTests: Orchestration, caching, preferences

Domain Model Tests:
- AchievementDefinitionsTests, AnySportTests, DivisionTests
- GameTests, ProgressTests, RegionTests, StadiumTests
- TeamTests, TravelSegmentTests, TripTests, TripPollTests
- TripPreferencesTests, TripStopTests, SportTests

Service Tests:
- FreeScoreAPITests, RouteDescriptionGeneratorTests
- SuggestedTripsGeneratorTests

Export Tests:
- ShareableContentTests (card types, themes, dimensions)

Bug fixes discovered through TDD:
- ShareCardDimensions: mapSnapshotSize exceeded available width (960x480)
- ScenarioBPlanner: Added anchor game validation filter

All tests include:
- Specification tests (expected behavior)
- Invariant tests (properties that must always hold)
- Edge case tests (boundary conditions)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Planning/Engine/ItineraryBuilder.swift

@@ -9,6 +9,11 @@
 import Foundation
 
 /// Result of building an itinerary from stops.
+///
+/// - Invariants:
+///   - travelSegments.count == stops.count - 1 (or 0 if stops.count <= 1)
+///   - totalDrivingHours >= 0
+///   - totalDistanceMiles >= 0
 struct BuiltItinerary {
     let stops: [ItineraryStop]
     let travelSegments: [TravelSegment]
@@ -17,6 +22,20 @@ struct BuiltItinerary {
 }
 
 /// Shared logic for building itineraries across all scenario planners.
+///
+/// Connects stops with travel segments and validates feasibility.
+///
+/// - Expected Behavior:
+///   - Empty stops → empty itinerary (no travel)
+///   - Single stop → single-stop itinerary (no travel)
+///   - Multiple stops → segments between each consecutive pair
+///   - Infeasible segment (exceeds driving limits) → returns nil
+///   - Validator returns false → returns nil
+///
+/// - Invariants:
+///   - travelSegments.count == stops.count - 1 for successful builds
+///   - All segments pass TravelEstimator feasibility check
+///   - All segments pass optional custom validator
 enum ItineraryBuilder {
 
     /// Validation that can be performed on each travel segment.
@@ -40,6 +59,15 @@ enum ItineraryBuilder {
     ///
     /// - Returns: Built itinerary if successful, nil if any segment fails
     ///
+    /// - Expected Behavior:
+    ///   - Empty stops → BuiltItinerary with empty segments
+    ///   - Single stop → BuiltItinerary with empty segments
+    ///   - Two stops → one segment connecting them
+    ///   - N stops → N-1 segments
+    ///   - TravelEstimator returns nil → returns nil
+    ///   - Validator returns false → returns nil
+    ///   - totalDrivingHours = sum of segment.estimatedDrivingHours
+    ///   - totalDistanceMiles = sum of segment.estimatedDistanceMiles
     static func build(
         stops: [ItineraryStop],
         constraints: DrivingConstraints,