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, 43 insertions, 0 deletions

diff --git a/src/features/surface-host/ui/Number.svelte b/src/features/surface-host/ui/Number.svelte
new file mode 100644
index 0000000..0f3323d
--- /dev/null
+++ b/src/features/surface-host/ui/Number.svelte
@@ -0,0 +1,43 @@
+<script lang="ts">
+	import type { InvokeMessage } from "@dispatch/ui-contract";
+	import type { NumberFieldView } from "../logic/types";
+
+	let {
+		field,
+		surfaceId,
+		onInvoke,
+	}: { field: NumberFieldView; surfaceId: string; onInvoke: (msg: InvokeMessage) => void } =
+		$props();
+
+	// Commit on change/Enter rather than every keystroke. Ignore empty/non-numeric
+	// input (the backend also floors/validates); send the new number as payload.
+	function commit(event: Event) {
+		const target = event.target as HTMLInputElement;
+		const next = target.valueAsNumber;
+		if (Number.isNaN(next)) return;
+		onInvoke({
+			type: "invoke",
+			surfaceId,
+			actionId: field.action.actionId,
+			payload: next,
+		});
+	}
+</script>
+
+<label class="flex items-center justify-between gap-2 text-sm">
+	<span>{field.label}</span>
+	<span class="flex items-center gap-1">
+		<input
+			type="number"
+			class="input input-bordered input-sm w-24"
+			value={field.value}
+			min={field.min}
+			max={field.max}
+			step={field.step}
+			onchange={commit}
+		/>
+		{#if field.unit}
+			<span class="opacity-60">{field.unit}</span>
+		{/if}
+	</span>
+</label>