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

📊 ~540 implementation lines, ~1,140 test lines

Extracts shared billing logic into billing-core.ts:
- consumeFromOrderedGrants: Core grant consumption algorithm
- getOrderedActiveGrants: Grant ordering and filtering
- calculateGrantCreditsToConsume: Per-grant consumption calculation

Includes comprehensive unit tests (160 tests) covering:
- Basic consumption scenarios
- Grant ordering (expiration, type priority)
- Boundary conditions and edge cases
- Negative balance handling

Fixes boundary condition: uses strict > comparison instead of >= for threshold checks.

Author: brandonkachen