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, 40 insertions, 0 deletions

diff --git a/src/features/markdown/ui/markdown.test.ts b/src/features/markdown/ui/markdown.test.ts
new file mode 100644
index 0000000..e34a4af
--- /dev/null
+++ b/src/features/markdown/ui/markdown.test.ts
@@ -0,0 +1,40 @@
+import { fireEvent, render, screen } from "@testing-library/svelte";
+import { describe, expect, it, vi } from "vitest";
+import Markdown from "./Markdown.svelte";
+
+describe("Markdown", () => {
+	it("renders markdown into a .markdown-body container", () => {
+		const { container } = render(Markdown, { props: { text: "# Hello\n\n**hi**" } });
+
+		expect(container.querySelector(".markdown-body")).not.toBeNull();
+		expect(screen.getByRole("heading", { level: 1, name: "Hello" })).toBeInTheDocument();
+		expect(container.querySelector("strong")?.textContent).toBe("hi");
+	});
+
+	it("strips dangerous markup", () => {
+		const { container } = render(Markdown, {
+			props: { text: "before <script>alert(1)</script> after" },
+		});
+
+		expect(container.querySelector("script")).toBeNull();
+		expect(container.textContent).toContain("before");
+	});
+
+	it("renders a copy button on a code block that copies the code to the clipboard", async () => {
+		const writeText = vi.fn().mockResolvedValue(undefined);
+		Object.defineProperty(navigator, "clipboard", { value: { writeText }, configurable: true });
+
+		const { container } = render(Markdown, {
+			props: { text: "```js\nconst x = 1;\n```" },
+		});
+
+		const button = container.querySelector<HTMLElement>("[data-copy]");
+		expect(button).not.toBeNull();
+		if (button === null) throw new Error("expected a copy button");
+
+		await fireEvent.click(button);
+
+		expect(writeText).toHaveBeenCalledTimes(1);
+		expect(writeText.mock.calls[0]?.[0]).toContain("const x = 1;");
+	});
+});