feat(simulation): add credit admission limits

[christopherjnelson]

Summary

• add execution-limits-v2 with atomic provider-attempt and credit-admission reservations
• retain conservative exposure for in-flight, cancelled, and unknown-cost calls while reconciling known costs exactly
• expose configuration and authoritative accounting in World Lab, with archived v1 compatibility and ADR coverage

Guarantee boundary

This is a server-side admission and exposure ceiling, not an upstream provider-account spending guarantee. When a credit ceiling is enabled, a reported cost above its configured reservation is recorded and future admission fails closed. Unlimited credit mode reports overruns without stopping execution.

Validation

Repository owner confirmed green:

• pnpm format
• pnpm validate
• pnpm test:e2e

Follow-up

O2b-2 will add a versioned independent provider-attempt ledger to safe exports and SQLite so cancelled and uncommitted calls are preserved for analysis.