Onboarding: funnel analytics + variant tagging for the new flow

[talissoncosta]

Part of #7540. Follows #7541, which mapped the current flow's analytics and agreed an activation metric set, explicitly leaving "instrument the new flow" to a separate ticket — this one.

Goal

Make the new single-page flow (#7737) comparable to the current onboarding in one funnel, broken down by variant — not a parallel set of events.

Work

• Both flows already emit the same milestone events from the shared stores (REGISTER, CREATE_FIRST_ORGANISATION, CREATE_FIRST_PROJECT, CREATE_FIRST_FEATURE), so the comparable skeleton exists.
• Tag those events with an onboarding_variant property (control | single_page), sourced from the FoF flag the flow is gated behind, so the funnel can be split by variant and compared directly.
• Ensure the activation milestone — first SDK evaluation received (Onboarding: track first SDK evaluation received #7623) — is captured and variant-tagged. It's the moment that matters.
• Add new-flow-only diagnostics (snippet.copied, AI-connect used, in-page toggle) for the single-page flow's internal drop-off — kept separate from the comparison funnel, since the control has no equivalent.
• Include org / project / environment IDs; no PII.

Sequencing

Instrumentation over the finished UI — wire it once the flow (#7737) has landed. One of the last tasks.