feat(rain): add fetchOrderBooks batch order-book fetch

[JSap0914]

What

RainExchange had a working single-outcome fetchOrderBook, but the batch
fetchOrderBooks fell through to the base class stub and threw
Method fetchOrderBooks not implemented.. exchange.has.fetchOrderBooks also
reported false.

This adds a fetchOrderBooks(outcomeIds) override that loops the already-working
fetchOrderBook and returns a Record<string, OrderBook> keyed by outcome id.

Why

Rain has no native batch order-book endpoint, so the batch variant is emulated
by fanning out to the single-outcome fetch — the same way fetchOrderBook
itself is already declared 'emulated'. To keep the capability map honest,
fetchOrderBooks is declared 'emulated' in capabilityOverrides (rather than
letting it auto-derive to true), so it stays consistent with the single fetch.

Tests

Added core/test/unit/rain-fetch-order-books.core.test.ts:

• fetchOrderBooks([...]) calls the single-outcome fetchOrderBook once per id
  (verified via a stub) and returns a record matching those N individual calls.
• exchange.has.fetchOrderBooks now reports 'emulated' instead of false.

# focused
npx jest -c jest.config.js test/unit/rain-fetch-order-books.core.test.ts
  Tests: 2 passed, 2 total

# full core suite
npx jest -c jest.config.js
  Test Suites: 48 passed, 1 skipped, 49 total
  Tests: 812 passed, 3 skipped, 815 total

# type check
npx tsc --noEmit   # exit 0

# generators (no drift)
node core/scripts/check-exchange-drift.js   # No drift. All layers in sync.
node core/scripts/generate-compliance.js    # COMPLIANCE.md unchanged

Fixes #1666