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, 13 insertions, 0 deletions

diff --git a/src/core/metrics/place.test.ts b/src/core/metrics/place.test.ts
index d94882d..0b9c0ec 100644
--- a/src/core/metrics/place.test.ts
+++ b/src/core/metrics/place.test.ts
@@ -526,4 +526,17 @@ describe("interleaveTurnMetrics — cumulative usage (cache total)", () => {
 		expect(tm[0]?.cumulativeUsage.inputTokens).toBe(1000);
 		expect(tm[0]?.cumulativeUsage.cacheReadTokens).toBe(500);
 	});
+
+	it("carries the prior finalized turn's usage as the retention baseline", () => {
+		const rows = interleaveTurnMetrics(
+			[userGroup(1, "q1"), assistantGroup(2, "a1"), userGroup(3, "q2"), assistantGroup(4, "a2")],
+			[cacheEntry("t1", 2669, 10, 384), cacheEntry("t2", 2737, 10, 2560)],
+		);
+		const tm = turnMetricsRows(rows);
+		// first finalized turn has no earlier baseline
+		expect(tm[0]?.prevTurnUsage).toBeNull();
+		// second turn's baseline is the first turn's usage
+		expect(tm[1]?.prevTurnUsage?.inputTokens).toBe(2669);
+		expect(tm[1]?.prevTurnUsage?.cacheReadTokens).toBe(384);
+	});
 });