M0.3 — Google-native first-party mode check

Status: blocked in this environment. Requires a live Vercel deployment and a real gtag-instrumented site to verify whether Google's own first-party mode for Google tags is usable on Vercel deployments. No deployed environment is available here.

What this blocks: scoping M3.1 (script-loading/proxying for gtag.js) around a supported mechanism instead of building parallel plumbing. Per the plan (1.7), this spike has no bearing on the Edge-vs-Node runtime choice for the collect handler — that's decided independently on latency/library-compatibility grounds, which US-007 does without needing this spike.

Default assumption taken in its absence: M3.1 (script proxying) stays out of v1 scope, as the plan already specifies — nothing in M1/M2 implementation depends on this spike's outcome.

To unblock: deploy apps/demo to a real Vercel project and test gtag('config', ..., { server_container_url: ... }) against it.