Add Laya as a local classification backend #75

Support @receptron/laya as an optional local alternative to TypeSafe/Jev for effort selection. Keep the existing request rewrite, fallback, and decision logging behavior.

• Map Laya’s typed answers to the router’s supported effort levels without changing the outbound model request contract.
• Make backend selection explicit. Preserve TypeSafe/Jev as the default and document Laya’s first-run model download, cache, and memory requirements.
• Test effort mapping, initialization failure, timeout, fallback, cancellation, and concurrent requests with a mocked Laya backend. Verify that local mode does not send conversation excerpts to TypeSafe/Jev.
• Smoke-test a real Laya model load and one routed request before calling support complete.