feat(audience-sdk): iOS SKAdNetwork registration on first launch (SDK-307)

[nattb8]

Summary

• Adds _AudienceRegisterSKAN() to the Obj-C++ bridge using runtime dispatch (NSClassFromString) to avoid a hard StoreKit link (which would trigger the In-App Purchase capability and block personal team builds)
• Calls registration on first launch when config.EnableMobileAttribution = true, skipping on subsequent launches via a PlayerPrefs flag
• Emits skanRegistered: true on game_launch only when registration fires (per-install signal — reappears after reinstall, not after Reset())
• Sample app gets a Mobile Attribution toggle on the Setup tab to drive EnableMobileAttribution

Depends on

PR #744 (SDK-308 — iOS Info.plist post-processor). Targets that branch so the diff is clean.

Test plan

• First launch with Mobile Attribution toggled on → game_launch contains skanRegistered: true
• Quit and relaunch → game_launch fires, skanRegistered absent
• SkanRegistrationTests — first launch calls bridge + persists, subsequent launch skips
• ImmutableAudienceTests — three new cases covering provider returns true/null and disabled flag

🤖 Generated with Claude Code

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Note

Medium Risk
Adds iOS native/Unity bridging and new game_launch telemetry fields gated by EnableMobileAttribution; risk centers on platform-conditional native interop and ensuring the new provider is only invoked when intended.

Overview
Adds opt-in iOS SKAdNetwork registration on first launch when AudienceConfig.EnableMobileAttribution is set, wiring a new Unity-installed MobileAttributionProvider into ImmutableAudience.Init and emitting skanRegistered: true on game_launch only when registration fires.

Introduces a runtime-dispatched Obj-C++ bridge (_AudienceRegisterSKAN) to avoid a hard StoreKit link, plus Unity-side SKANBridge/SkanRegistration (PlayerPrefs-backed “first launch” flag). Updates the sample app UI to expose a Mobile Attribution toggle and expands tests to cover provider behavior and first-launch persistence/ordering.

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