feat(surfaces): NumberField + per-conversation surface scoping; cache-warming controls

Extend the surface framework so cache-warming exposes per-conversation controls:
- ui-contract: add NumberField (settable free-value numeric) to SurfaceField;
  add optional conversationId to subscribe/unsubscribe/invoke + surface/update
- surface-registry: SurfaceContext { conversationId? } on getSpec/invoke (backward-compatible)
- transport-ws: thread conversationId; key subscriptions by (surfaceId, conversationId);
  tag surface/update replies with conversationId
- cache-warming: per-conversation surface — Toggle(enabled) + Number(interval seconds,
  cache-warming/set-interval) + Stat(last cache %); drop the currentConversationId closure

Global surfaces (surface-loaded-extensions) unchanged. 784 vitest + 109 bun = 893 tests;
tsc -b EXIT 0; biome clean.

1 files changed, 1 insertions, 1 deletions

diff --git a/packages/surface-registry/src/index.ts b/packages/surface-registry/src/index.ts
index cdfcf7e..da52c92 100644
--- a/packages/surface-registry/src/index.ts
+++ b/packages/surface-registry/src/index.ts
@@ -1,4 +1,4 @@
 export { createSurfaceRegistryExtension, manifest } from "./extension.js";
-export type { SurfaceProvider, SurfaceRegistry } from "./registry.js";
+export type { SurfaceContext, SurfaceProvider, SurfaceRegistry } from "./registry.js";
 export { createSurfaceRegistry } from "./registry.js";
 export { surfaceRegistryHandle } from "./service.js";