refactor(billing): consolidate billing duplication (Commit 2.4)

Creates billing-core.ts with shared billing logic:
- getOrderedActiveGrantsForOwner: Generic grant fetching for user/org
- calculateUsageAndBalanceFromGrants: Usage and balance calculation
- GRANT_ORDER_BY: Consistent grant ordering constant
- CreditBalance, CreditUsageAndBalance interfaces

Fixes critical bug in consumeFromOrderedGrants:
- Tracks effectiveBalances Map to prevent stale balance reads during multi-pass consumption

Comprehensive tests with:
- Simplified logger assertions (check calls, not specific messages)
- Removed trivial edge cases (zero/empty/fractional tests)
- Consolidated mock setup (shared beforeEach)

Author: brandonkachen