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, 41 insertions, 0 deletions

diff --git a/src/features/surface-host/logic/plan.test.ts b/src/features/surface-host/logic/plan.test.ts
index a5727b4..be296a7 100644
--- a/src/features/surface-host/logic/plan.test.ts
+++ b/src/features/surface-host/logic/plan.test.ts
@@ -57,6 +57,47 @@ describe("planSurface", () => {
 		expect(plan.fields).toEqual([{ kind: "stat", label: "Tokens", value: "1,234" }]);
 	});
 
+	it("maps a number field to a NumberFieldView, carrying optional hints", () => {
+		const plan = planSurface(
+			makeSpec({
+				kind: "number",
+				label: "Interval",
+				value: 240,
+				min: 1,
+				step: 1,
+				unit: "s",
+				action: { actionId: "cache-warming/set-interval" },
+			}),
+		);
+		expect(plan.fields).toEqual([
+			{
+				kind: "number",
+				label: "Interval",
+				value: 240,
+				min: 1,
+				step: 1,
+				unit: "s",
+				action: { actionId: "cache-warming/set-interval" },
+			},
+		]);
+	});
+
+	it("omits absent number hints (no max key when undefined)", () => {
+		const plan = planSurface(
+			makeSpec({
+				kind: "number",
+				label: "Interval",
+				value: 240,
+				min: 1,
+				action: { actionId: "set" },
+			}),
+		);
+		const field = plan.fields[0];
+		expect(field).not.toHaveProperty("max");
+		expect(field).not.toHaveProperty("step");
+		expect(field).not.toHaveProperty("unit");
+	});
+
 	it("maps a button field to a ButtonFieldView", () => {
 		const plan = planSurface(
 			makeSpec({ kind: "button", label: "Retry", action: { actionId: "retry" } }),