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.

2 files changed, 12 insertions, 3 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";
diff --git a/packages/surface-registry/src/registry.ts b/packages/surface-registry/src/registry.ts
index b1c8116..5780910 100644
--- a/packages/surface-registry/src/registry.ts
+++ b/packages/surface-registry/src/registry.ts
@@ -1,6 +1,15 @@
 import type { SurfaceCatalog, SurfaceCatalogEntry, SurfaceSpec } from "@dispatch/ui-contract";
 
 /**
+ * Optional context threaded by the transport when calling a surface provider.
+ * Providers may use this to scope per-conversation state; omitting it yields
+ * the default/global behaviour.
+ */
+export interface SurfaceContext {
+	readonly conversationId?: string;
+}
+
+/**
  * What a surface-contributing extension registers with the surface registry.
  * Each provider owns one surface identified by its catalog entry id.
  */
@@ -9,10 +18,10 @@ export interface SurfaceProvider {
 	readonly catalogEntry: SurfaceCatalogEntry;
 
 	/** Build the current surface spec (may be async for dynamic surfaces). */
-	getSpec(): SurfaceSpec | Promise<SurfaceSpec>;
+	getSpec(context?: SurfaceContext): SurfaceSpec | Promise<SurfaceSpec>;
 
 	/** Run a backend action by id with an optional payload. */
-	invoke(actionId: string, payload?: unknown): void | Promise<void>;
+	invoke(actionId: string, payload?: unknown, context?: SurfaceContext): void | Promise<void>;
 
 	/**
 	 * Optional: subscribe to spec changes. Returns an unsubscribe disposer.