#1703 docs: record platform runtime adoption checkpoint

Summary

Record ADR 0019 checkpoint outcome as revise.

• Preserve the accepted, coherent devices, logs, and network cutovers.
• Record passing architecture, behavior, lifetime, laziness, startup, and coupling evidence.
• Record the hard package-size failure and prohibit broader migration pending an explicit packaging/budget decision and clean checkpoint rerun.

Stacked on #1702. Tracks #1696. Follow-up decision: #1704.

Validation

• pnpm check:affected --run (3,968 tests; 87.70% changed-line coverage)
• controlled same-host pnpm build + node scripts/size-report.mjs --startup-runs 15 baseline/checkpoint comparison
• final adversarial claude -p review; confirmed network line-offset P1 fixed in #1702, no remaining P0/P1 findings