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, 31 insertions, 2 deletions

diff --git a/src/features/surface-host/logic/plan.ts b/src/features/surface-host/logic/plan.ts
index 769f9f9..89088c3 100644
--- a/src/features/surface-host/logic/plan.ts
+++ b/src/features/surface-host/logic/plan.ts
@@ -1,7 +1,21 @@
 import type { InvokeMessage, SurfaceSpec } from "@dispatch/ui-contract";
-import type { FieldView, RenderGroup, StatFieldView, SurfaceRenderPlan } from "./types";
+import type {
+	FieldView,
+	NumberFieldView,
+	RenderGroup,
+	StatFieldView,
+	SurfaceRenderPlan,
+} from "./types";
 
-const KNOWN_KINDS = new Set(["toggle", "progress", "selector", "stat", "button", "custom"]);
+const KNOWN_KINDS = new Set([
+	"toggle",
+	"progress",
+	"selector",
+	"stat",
+	"number",
+	"button",
+	"custom",
+]);
 
 /**
  * Validate and normalise a SurfaceSpec into a renderable plan.
@@ -46,6 +60,21 @@ export function planSurface(spec: SurfaceSpec): SurfaceRenderPlan {
 					value: field.value,
 				});
 				break;
+			case "number": {
+				// Carry optional hints only when present (exactOptionalPropertyTypes).
+				const view: NumberFieldView = {
+					kind: "number",
+					label: field.label,
+					value: field.value,
+					action: field.action,
+					...(field.min !== undefined ? { min: field.min } : {}),
+					...(field.max !== undefined ? { max: field.max } : {}),
+					...(field.step !== undefined ? { step: field.step } : {}),
+					...(field.unit !== undefined ? { unit: field.unit } : {}),
+				};
+				fields.push(view);
+				break;
+			}
 			case "button":
 				fields.push({
 					kind: "button",