## Changes
Introduces two new files in `packages/common`:
- **`telemetry-first-touch-store.ts`** — a module-scoped singleton that
holds first-touch attribution data (referrer, UTM params, page URL) in
memory. Writes once on first load, cleared after the initial pageview
event fires or on opt-out. No device storage involved.
- **`useFirstTouchStore.tsx`** — a React hook that captures attribution
data on initial page load and writes it into the store, gated on the
`enabled` flag so it only runs where consent has been handled.
Trade-off: data is lost on a hard reload before consent is granted —
accepted edge case per GROWTH-656.
Follows the same module-scope pattern already used by `posthogClient`
and `consentState`.
## Testing
- Verify first-touch data is captured on initial load and readable by
`PageTelemetry` after consent
- Verify no cookie is set before consent
- Verify data is cleared after initial pageview fires
GROWTH-656
