feat(audience): auto-collect Epic account ID and detect Epic platform

[nattb8]

Summary

• When the EOS Unity plugin (PlayEveryWare.EpicOnlineServices) is present and EOSManager has been initialised by the game, the SDK now automatically calls Identify() with the logged-in ProductUserId at Init
• Sets distribution_platform = "epic" in game_launch properties when EOS is active
• Detection uses C# reflection — no hard compile-time dependency on EOS; games without EOS are completely unaffected
• link.xml entries for EOSSDK and PlayEveryWare.EpicOnlineServices prevent IL2CPP from stripping the types reached via reflection; absent assemblies are silently ignored

The reflection chain: EOSManager.Instance → GetEOSPlatformInterface() → GetConnectInterface() → GetLoggedInUserByIndex(0) → ProductUserId.ToString()

Notes for docs (SDK-536)

• EOS must be initialised via EOSManager before ImmutableAudience.Init() is called — the SDK reads the logged-in state at Init time; late EOS init will not be picked up
• If the game uses the raw EOS C SDK without the PlayEveryWare Unity plugin, auto-collect will not fire; studios must call Identify() manually
• distribution_platform can be overridden via AudienceConfig.DistributionPlatform; the config value always wins over auto-detection

Test plan

• Verified full reflection chain resolves under IL2CPP on macOS (arm64) using mock EOS DLLs
• auto-identified epic user: <id> log line appears when EOS is initialised
• distribution_platform = "epic" set in game_launch when EOS platform interface is available
• No log output (no warn, no crash) when EOS is absent — Steam-only or no-platform games unaffected

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