Add Polar billing emulator and release 0.15.0

[joelbqz]

Adds @emulators/polar (npx emulate --service polar): a stateful emulator for the subscription and usage-based billing surface applications use through @polar-sh/sdk.

• Customers with external ids (Polar's exact 422 messages for duplicate email / external id), meters with filters and aggregations, event ingestion, meter credit and custom benefits, products with fixed or metered prices and trials.
• Subscriptions: server-side creation for free products only, product changes with next_period pending updates, cancel at period end, revoke; customer state with active subscriptions, granted benefits, and meter balances; customer meters.
• Hosted checkout that upgrades a free subscription in place via subscription_id, with settlement deferred until POST /checkout/:secret/settle (or an auto-settle delay), and customer portal sessions.
• Faults and the request ledger key on Polar's real operation ids (customers:get_state_external, events:ingest, ...).
• Registry entry, skill, docs, README, and a test suite driving the real SDK against the emulator.
• Version bump to 0.15.0 with the changelog entry, so merging publishes.

Verified: turbo run build type-check lint test --filter=@emulators/polar --filter=@executor-js/emulate --filter=web passes, and the executor cloud billing e2e scenarios pass against this emulator.

🤖 Generated with Claude Code

https://claude.ai/code/session_01EjMMoUk6xckQ7FX87nrNdZ