feat(arena-colosseum): playable bracket + rank-match promotion (phase 2)

Closes 4 more arena-colosseum spec shapes (6/16 → 10/16): the dual-prefix
battle URLs (colosseum_battle/* and colosseum_rank_battle/*) for both
do_matching and per-match finish, plus the bracket-end /finish + /retire
pair on /arena_colosseum.

* ColosseumProgressionService: pure-logic decisions for win-threshold
  advancement, 3008 promotion trigger, and per-round reward bundles.
  Extends ColosseumRoundsConfig with FinishRewards/RetireRewards per
  round + a top-level ChampionRewards list.
* MatchContextBuilder.BuildForColosseumAsync: lifts the registered deck
  via IDeckRepository (NOT viewer-graph) per project_ef_nav_include_pitfall.
* ArenaColosseumBattleController: dual route prefixes off one controller
  via explicit absolute [HttpPost("/...")] routes — does NOT extend
  SVSimController's [Route("[controller]")]. Same pattern as FreeBattle.
  URL-vs-IsRankMatching mismatch returns 400 with a phase-mismatch error;
  no-deck triggers the standard 3001 matchmaking-illegal state. 3008
  flips run.IsRankMatching exactly once via the progression service.
* /arena_colosseum/finish + /retire share FinishResponse (rewards +
  reward_list + colosseum_status). Champion finish appends ChampionRewards
  and surfaces colosseum_status.is_champion = true. Both delete the run
  row after granting via IInventoryTransaction.GrantAsync.
* DI: ModePolicy entries for "colosseum_battle" and "colosseum_rank_battle".
* Tests: 8 progression-service unit tests (pure logic), 5 battle-controller
  HTTP tests, 5 bracket-terminate HTTP tests, 2 full-bracket E2E walks
  (champion path + retire mid-round). Full suite: 1331/1331.

SVSim.EmulatedEntrypoint/Services/IMatchContextBuilder.cs

@@ -24,4 +24,13 @@ public interface IMatchContextBuilder
     /// no deck at that slot.
     /// </summary>
     Task<MatchContext> BuildForRankBattleAsync(long viewerId, Format format, int deckNo);
+
+    /// <summary>
+    /// Build a context for an Arena Colosseum bracket match — reads the active
+    /// <c>ViewerArenaColosseumRun</c>'s registered deck slot (single-deck v1) via
+    /// <c>IDeckRepository.GetDeck</c> (NOT viewer-graph traversal per
+    /// <c>project_ef_nav_include_pitfall</c>). Throws when the run is missing or no deck
+    /// has been registered yet.
+    /// </summary>
+    Task<MatchContext> BuildForColosseumAsync(long viewerId);
 }