[META] Phase 7: Multi-vendor

## Parent

Sub-issue of https://github.com/elastic/obs-integration-team/issues/1048 (Phase 7 — Multi-vendor).

## Goal

Extend the migration platform to additional observability vendors after the Grafana and Datadog foundations are stable.

## Target

H2 2026.

## Exit Criteria

- New vendor onboarding has a repeatable adapter contract.
- New Relic and Splunk scope is defined before implementation starts.
- Vendor-specific semantic gaps are tracked explicitly.

## Filing

No program meta exists for extra vendors yet. When this work starts, parent New Relic / Splunk issues here until a program is added under #1048.