feat(rank-battle): real DoMatching with PvP pair + AI fallback mapping

DoMatchingInternal calls IMatchingPairUpService.TryPairAsync, then maps:
- null result → 3002 RETRY (empty node_server_url, no battle_id)
- IsAiFallback → 3011 AI_BATTLE_MATCHING_SUCCEEDED
- IsOwner → 3007 SUCCEEDED_OWNER (cache pickup)
- joiner → 3004 SUCCEEDED

BuildForRankBattleAsync's InvalidOperationException (typically "no deck
for format") surfaces as 3001 ILLEGAL so the client shows the
matchmaking-error dialog rather than retrying.

card_master_id is a placeholder (0) per the per-battle card-master
split deferral. AI-fallback timing is covered by InProcessPairUp unit
tests; controller tests focus on the wire mapping (3002, 3004, 3007).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Controllers/RankBattleController.cs

@@ -107,12 +107,54 @@ public sealed class RankBattleController : ControllerBase
         return Ok(new { });
     }
 
-    // Filled in by Task 9.
-    private Task<IActionResult> DoMatchingInternal(string mode, Format format, DoMatchingRequestDto req, CancellationToken ct)
+    private async Task<IActionResult> DoMatchingInternal(string mode, Format format, DoMatchingRequestDto req, CancellationToken ct)
     {
-        if (!TryGetViewerId(out var _)) return Task.FromResult<IActionResult>(Unauthorized());
-        // Placeholder; real impl arrives in Task 9.
-        return Task.FromResult<IActionResult>(Ok(new DoMatchingResponseDto { MatchingState = 3002 }));
+        if (!TryGetViewerId(out var vid)) return Unauthorized();
+
+        MatchContext ctx;
+        try
+        {
+            ctx = await _ctxBuilder.BuildForRankBattleAsync(vid, format);
+        }
+        catch (InvalidOperationException ex)
+        {
+            // Most likely cause: viewer has no deck for this format. Surface as 3001
+            // RC_BATTLE_MATCHING_ILLEGAL — the client shows the standard matchmaking-error
+            // dialog rather than retrying forever.
+            _log.LogWarning(ex, "BuildForRankBattleAsync failed for viewer {Vid} format {Fmt}; returning 3001.", vid, format);
+            return Ok(new DoMatchingResponseDto { MatchingState = 3001, NodeServerUrl = "" });
+        }
+
+        var paired = await _pairUp.TryPairAsync(mode, new BattlePlayer(vid, ctx), ct);
+
+        if (paired is null)
+        {
+            // Parked. 3002 RETRY. node_server_url must be present as empty string —
+            // client's DoMatchingBase parser calls .ToString() without a guard.
+            return Ok(new DoMatchingResponseDto
+            {
+                MatchingState = 3002,
+                NodeServerUrl = "",
+            });
+        }
+
+        // Owner cache-pickup → 3007 (PvP) or 3011 (AI fallback).
+        // Joiner (only PvP) → 3004.
+        var state = paired switch
+        {
+            { IsAiFallback: true } => 3011,
+            { IsOwner: true } => 3007,
+            _ => 3004,
+        };
+
+        return Ok(new DoMatchingResponseDto
+        {
+            MatchingState = state,
+            BattleId = paired.Match.BattleId,
+            NodeServerUrl = paired.Match.NodeServerUrl,
+            // Placeholder per spec § Out of scope — per-battle card-master split is deferred.
+            CardMasterId = 0,
+        });
     }
 
     // Filled in by Task 10.