chore(med): mapping culture-safe parse, dead-code, scope comparer, UA rotator, parser cache

Six MEDIUM-tier review items:

* Mapping.cs — DateTimeOffset.Parse now passes CultureInfo.InvariantCulture
  + DateTimeStyles.RoundtripKind so a non-en-US thread culture cannot
  corrupt round-tripped ScheduledAt / CapturedAt / DetectedAt / CompletedAt.
  Also replaces the magic 0/1 BetScope discriminator with named constants.

* Delete dead Placeholder.cs files in Marathon.Application and
  Marathon.Infrastructure — they were stubs from Phase 1 to satisfy
  "non-empty project" and have been dead since Phase 2/3.

* EventBrowsingService — drop the bespoke ScopeEqualityComparer; BetScope
  is a record hierarchy, .GroupBy uses value equality natively.

* UserAgentRotatorHandler — counter promoted to private static int with
  Interlocked.Increment so rotation is round-robin across the process.
  HttpClientFactory builds the handler Transient, so the previous instance
  field reset to zero on every new client and broke rotation.

* EventOddsParser — added a parallel "selection-key → IElement" index
  alongside the existing price index. Handicap extraction (6 call sites
  per event detail page) used to do a fresh document.QuerySelector("span[
  data-selection-key='...']") for every key — full-document CSS traversal.
  Now it's a dictionary lookup, with the pair-emit logic factored into a
  shared TryEmitHandicapPair helper.

src/Marathon.UI/Services/EventBrowsingService.cs

@@ -204,8 +204,10 @@ public sealed class EventBrowsingService : IEventBrowsingService
     private static IReadOnlyList<EventScopeBoard> BuildBoards(OddsSnapshot snapshot)
     {
         // Group by scope, preserve Match-first order then ascending Period numbers.
+        // BetScope is a record hierarchy so .GroupBy uses value equality natively —
+        // no custom comparer needed.
         var groups = snapshot.Bets
-            .GroupBy(static b => b.Scope, ScopeEqualityComparer.Instance)
+            .GroupBy(static b => b.Scope)
             .OrderBy(static g => OrderKey(g.Key));
 
         var boards = new List<EventScopeBoard>();
@@ -241,23 +243,4 @@ public sealed class EventBrowsingService : IEventBrowsingService
         PeriodScope p => p.Number,
         _ => int.MaxValue,
     };
-
-    private sealed class ScopeEqualityComparer : IEqualityComparer<BetScope>
-    {
-        public static readonly ScopeEqualityComparer Instance = new();
-        public bool Equals(BetScope? x, BetScope? y) => (x, y) switch
-        {
-            (null, null) => true,
-            (MatchScope, MatchScope) => true,
-            (PeriodScope a, PeriodScope b) => a.Number == b.Number,
-            _ => false,
-        };
-
-        public int GetHashCode(BetScope obj) => obj switch
-        {
-            MatchScope => 0,
-            PeriodScope p => p.Number,
-            _ => -1,
-        };
-    }
 }