feat(audience): auto-collect platform ID on consent upgrade to Full

[nattb8]

Summary

Previously Steam and Epic identity collection only ran at Init. Games that start with Anonymous consent and upgrade to Full via SetConsent never had the platform ID collected, leaving a gap in identity data for the common consent-prompt flow.

The fix adds TryIdentifySteamUser and TryIdentifyEpicUser to SetConsent, gated on a genuine upgrade to Full (level == Full && previous != Full). The same two methods already run at Init, so behaviour is consistent across both entry points.

Closes SDK-539 / part of SDK-534.

Use case gotchas

If a game calls Identify with its own backend user ID and then cycles consent (Full → Anonymous → Full), auto-collection fires again on the upgrade and sets the platform ID as the active user ID. The dev needs to call Identify again with their game user ID after the upgrade — this is expected behaviour since the downgrade already cleared identity state.

If the platform session is not active at the moment consent reaches Full (Steam not running, EOS not yet logged in), collection is silently skipped. Devs who need identity reliably in that scenario should call Identify manually once the platform session is ready.

Test plan

• Consent starts Anonymous, upgraded to Full via SetConsent — identify event with identity_type='steam' fires
• Consent starts Anonymous, upgraded to Full via SetConsent — identify event with identity_type='epic' fires
• Redundant SetConsent(Full) when already Full — auto-collection does not fire again
• No errors when neither Steam nor EOS plugin is present

🤖 Generated with Claude Code

[nattb8]

Reverting — firing on consent upgrade adds DX complexity for limited gain. The platform session (EOS login especially) is often not ready at the moment SetConsent is called, so it silently skips anyway. The consent-cycle case also creates an identity overwrite trap. Init-only is simpler and more predictable.