docs(run-db-split): add Wave-0 PlanetScale run-ops provisioning runbook (W0-FND-04)

Authored as a docs-only runbook: captures the PlanetScale PG17 run-ops DB
provisioning procedure, inter-provider networking, reachability proof,
cross-provider latency-budget measurement, the canonical run-ops env-var
contract, env wiring, and rollback/teardown.

Marked STATUS: NOT YET EXECUTED — live PlanetScale/AWS provisioning and
verification are deferred to a human operator. No infrastructure was
provisioned and no secrets are recorded; all live values are placeholders.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: d-cs