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.Application/Placeholder.cs

@@ -1,5 +0,0 @@
-// Phase 2/3/4 will populate this project.
-// This file exists only to prevent the compiler from treating the project as empty.
-namespace Marathon.Application;
-
-internal static class Placeholder { }