feat(cache-warming,surfaces,metrics,markdown): conversation-scoped surfaces, cache warming + retention, markdown

Consumes the backend cache-warming + cache-rate handoffs end-to-end and adds supporting infra:

- protocol/transport: conversation-scoped surfaces (conversationId on subscribe/invoke/surface + staleness routing); store auto-subscribes the catalog with the focused conversation and re-scopes on switch.
- surface-host: generic Number field renderer + custom rendererId dispatch (graceful skip on unknown).
- cache-warming feature: enabled toggle, min+sec interval, AUTHORITATIVE countdown from the surface's cache-warming-timer nextWarmAt, manual Warm now (POST /chat/warm), lastWarmAt-keyed history, cache-retention stat, expectedCacheRate headline.
- metrics: cross-turn expected-cache (retention) derivation + bubble badge; cache-rate fix needs no code change (inputTokens now total).
- markdown feature: marked + marked-highlight + highlight.js + dompurify, rendered in ChatView.
- fixes (gemini review): {#key activeConversationId} remount of CacheWarmingView to stop history/feedback leaking across tabs; guard NaN interval inputs from committing 0.
- docs/contracts: regenerated transport/ui-contract mirrors; backend-handoff updated (CR-3 resolved).

Verified: svelte-check 0 errors, biome clean, 494 tests pass, vite build OK.

1 files changed, 9 insertions, 2 deletions

diff --git a/src/core/protocol/index.ts b/src/core/protocol/index.ts
index 25174ea..e7fd161 100644
--- a/src/core/protocol/index.ts
+++ b/src/core/protocol/index.ts
@@ -1,2 +1,9 @@
-export { applyServerMessage, initialState, invoke, subscribe, unsubscribe } from "./reducer";
-export type { ProtocolResult, ProtocolState } from "./types";
+export {
+	applyServerMessage,
+	getSurfaceSpec,
+	initialState,
+	invoke,
+	subscribe,
+	unsubscribe,
+} from "./reducer";
+export type { ProtocolResult, ProtocolState, Subscription } from "./types";