feat(#1683): published Host-Integration SDK surface + smoke test — Slice 2 (#1939)

The SDK entry (src/host-integration-sdk.cts) is the single PUBLIC surface a
host-plugin author imports: the negotiated schema + classification, the five
adapters (declarative/imperative/model/hook/state), and the serialized
handshake. Frozen so the public shape cannot be mutated. Everything else in
gsd-core stays internal.

tests/sdk-smoke.test.cjs imports ONLY from the SDK entry and builds a
third-party host-plugin end-to-end (compose adapters + handshake + classify) —
proving an external author can wire a host without gsd-core internals (#1683 AC).
ESLint-ignore + inventory manifest kept in sync for the new tsc-emitted module.

docs/INVENTORY-MANIFEST.json

@@ -341,6 +341,7 @@
       "handshake-serialized.cjs",
       "hook-bus.cjs",
       "host-integration.cjs",
+      "host-integration-sdk.cjs",
       "init-command-router.cjs",
       "init.cjs",
       "install-engine.cjs",