fix(audience): prevent IL2CPP stripping of Unity hooks assembly

[nattb8]

Fixes SDK-480.

Problem

Games built with IL2CPP + High stripping silently lose all Unity context fields (userAgent, locale, screen, timezone) from every event. The linker strips the entire Immutable.Audience.Unity assembly when it finds no call sites into it — so the [RuntimeInitializeOnLoadMethod] hook never fires and ContextProvider is never registered. Only library and libraryVersion survive because they come from Immutable.Audience.Runtime, which has direct references keeping it alive.

The SDK ships a link.xml to guard against this, but Unity's linker skips it for packages loaded via a file: path outside the project tree. Studios without a project-level link.xml fallback hit this silently.

Confirmed in production on project 18282: ~97% of 0.3.0 events are missing userAgent. All 0.2.2 events from the same project had it — the regression correlates with them switching to an IL2CPP + High stripping build around the 0.3.0 upgrade.

Fix

Two lines in AudienceUnityHooks.cs:

• [assembly: AlwaysLinkAssembly] — forces the linker to process the assembly regardless of call sites
• [Preserve] on the class — keeps Install() alive once the assembly is included

[Preserve] alone is not enough: if the linker skips the assembly entirely, it never evaluates the attribute.

Test plan

• Build example app with IL2CPP + High stripping and no audience lines in link.xml
• Confirm events include full context (userAgent, locale, screen, timezone)
• Confirm no PersistentDataPath is required crash on init