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, 165 insertions, 0 deletions

diff --git a/src/features/markdown/logic/markdown.ts b/src/features/markdown/logic/markdown.ts
new file mode 100644
index 0000000..3a6e5a6
--- /dev/null
+++ b/src/features/markdown/logic/markdown.ts
@@ -0,0 +1,165 @@
+/**
+ * Pure Markdown → sanitized-HTML renderer for assistant messages.
+ *
+ * Mirrors old Dispatch's stack (marked + marked-highlight + highlight.js +
+ * DOMPurify; GFM + line breaks; streaming delimiter-closing), but kept fully
+ * SYNCHRONOUS and pure: `input → output`, no effects, no `$effect`. Languages
+ * are a fixed "hot set" registered at module load (no lazy dynamic import), so a
+ * single `renderMarkdown(text)` call is deterministic and unit-testable.
+ *
+ * The only ambient dependency is DOMPurify, which sanitizes against the DOM —
+ * present in the browser and in the jsdom test env.
+ */
+
+import DOMPurify from "dompurify";
+import type { LanguageFn } from "highlight.js";
+import hljs from "highlight.js/lib/core";
+import bash from "highlight.js/lib/languages/bash";
+import c from "highlight.js/lib/languages/c";
+import cpp from "highlight.js/lib/languages/cpp";
+import csharp from "highlight.js/lib/languages/csharp";
+import css from "highlight.js/lib/languages/css";
+import go from "highlight.js/lib/languages/go";
+import java from "highlight.js/lib/languages/java";
+import javascript from "highlight.js/lib/languages/javascript";
+import json from "highlight.js/lib/languages/json";
+import markdownLang from "highlight.js/lib/languages/markdown";
+import php from "highlight.js/lib/languages/php";
+import plaintext from "highlight.js/lib/languages/plaintext";
+import python from "highlight.js/lib/languages/python";
+import ruby from "highlight.js/lib/languages/ruby";
+import rust from "highlight.js/lib/languages/rust";
+import shell from "highlight.js/lib/languages/shell";
+import sql from "highlight.js/lib/languages/sql";
+import typescript from "highlight.js/lib/languages/typescript";
+import xml from "highlight.js/lib/languages/xml";
+import yaml from "highlight.js/lib/languages/yaml";
+import { Marked } from "marked";
+import { markedHighlight } from "marked-highlight";
+
+// Hot set: registered eagerly so common code blocks highlight on first paint.
+const HOT_LANGUAGES: Record<string, LanguageFn> = {
+	bash,
+	c,
+	cpp,
+	csharp,
+	css,
+	go,
+	java,
+	javascript,
+	json,
+	markdown: markdownLang,
+	php,
+	plaintext,
+	python,
+	ruby,
+	rust,
+	shell,
+	sql,
+	typescript,
+	xml,
+	yaml,
+};
+for (const [name, lang] of Object.entries(HOT_LANGUAGES)) {
+	hljs.registerLanguage(name, lang);
+}
+
+// Normalize common fence aliases to canonical highlight.js names.
+const ALIASES: Record<string, string> = {
+	js: "javascript",
+	jsx: "javascript",
+	mjs: "javascript",
+	cjs: "javascript",
+	ts: "typescript",
+	tsx: "typescript",
+	py: "python",
+	py3: "python",
+	rb: "ruby",
+	sh: "bash",
+	zsh: "bash",
+	yml: "yaml",
+	"c++": "cpp",
+	cxx: "cpp",
+	"c#": "csharp",
+	cs: "csharp",
+	htm: "xml",
+	html: "xml",
+	svg: "xml",
+	md: "markdown",
+	mdx: "markdown",
+	golang: "go",
+	rs: "rust",
+};
+
+function normalizeLang(lang: string): string {
+	const lower = lang.toLowerCase().trim();
+	return ALIASES[lower] ?? lower;
+}
+
+function escapeHtml(s: string): string {
+	return s
+		.replace(/&/g, "&amp;")
+		.replace(/</g, "&lt;")
+		.replace(/>/g, "&gt;")
+		.replace(/"/g, "&quot;")
+		.replace(/'/g, "&#39;");
+}
+
+const md = new Marked(
+	markedHighlight({
+		emptyLangClass: "hljs",
+		langPrefix: "hljs language-",
+		highlight(code: string, lang: string): string {
+			if (!lang) return escapeHtml(code);
+			const name = normalizeLang(lang);
+			if (!hljs.getLanguage(name)) return escapeHtml(code);
+			try {
+				return hljs.highlight(code, { language: name, ignoreIllegals: true }).value;
+			} catch {
+				return escapeHtml(code);
+			}
+		},
+	}),
+	{ gfm: true, breaks: true },
+);
+
+/**
+ * While a message is still streaming, balance dangling fences / emphasis so the
+ * partial text renders cleanly instead of flashing raw markers.
+ */
+function closeOpenDelimiters(src: string): string {
+	let out = src;
+	const fenceCount = (out.match(/^```/gm) ?? []).length;
+	if (fenceCount % 2 !== 0) out += "\n```";
+	const boldCount = (out.match(/\*\*/g) ?? []).length;
+	if (boldCount % 2 !== 0) out += "**";
+	const inlineCode = (out.match(/(?<!`)`(?!`)/g) ?? []).length;
+	if (inlineCode % 2 !== 0) out += "`";
+	return out;
+}
+
+// Wrap each fenced code block (`<pre>…</pre>`) in a positioned container with a
+// copy button. marked emits exactly one `<pre>`/`</pre>` pair per block and
+// escapes `<`/`>` inside code, so these literal tags only ever delimit blocks.
+// `data-copy` is the delegation hook the component listens for; DOMPurify keeps
+// `<button>` + `data-*` by default. Inline `<code>` has no `<pre>`, so it's untouched.
+const COPY_BUTTON =
+	'<button type="button" data-copy aria-label="Copy code"' +
+	' class="copy-btn btn btn-xs absolute right-2 top-2 opacity-0 transition-opacity group-hover:opacity-100">Copy</button>';
+
+function addCopyButtons(html: string): string {
+	return html
+		.replace(/<pre>/g, `<div class="code-block group relative">${COPY_BUTTON}<pre>`)
+		.replace(/<\/pre>/g, "</pre></div>");
+}
+
+/** Render Markdown to sanitized HTML. Returns `""` if parsing ever throws. */
+export function renderMarkdown(text: string, opts?: { streaming?: boolean }): string {
+	const src = opts?.streaming === true ? closeOpenDelimiters(text) : text;
+	try {
+		const raw = md.parse(src) as string;
+		return DOMPurify.sanitize(addCopyButtons(raw));
+	} catch {
+		return "";
+	}
+}