perf(web-read-card): lazy-load read grammars and guard empty-window copy

Only TypeScript, shell, and JSON grammars load at Web boot; the read card's
wider langFromPath extension set loads through dynamic imports on first use,
so a session that never opens a read card in one of those languages avoids
~1.6 MB of grammar modules and their synchronous init. ReadBlock/CodeBlock
re-render on grammar-load via useSyncExternalStore, picking up highlighting
once the grammar registers. ReadBlock hides the copy control on an empty
window (a successful read of an empty file settles to lines: [] with
card:'read'), matching TerminalBlock so it cannot wipe the clipboard.

packages/client/ui-primitives/src/markdown/CodeBlock.tsx

@@ -4,10 +4,10 @@
 // plain fallback for everything else. Chrome (language banner + copy) matches
 // deepsuite `@deepseek/md` code blocks; token colors stay on `--shiki-*`.
 
-import { useCallback, useMemo, useRef, useState } from 'react'
+import { useCallback, useMemo, useRef, useState, useSyncExternalStore } from 'react'
 import clsx from 'clsx'
 import { writeClipboard } from '../clipboard.ts'
-import { highlightToHtml } from './highlight.ts'
+import { grammarLoadCount, highlightToHtml, subscribeGrammarLoaded } from './highlight.ts'
 import css from './CodeBlock.module.css'
 
 export interface CodeBlockProps {
@@ -21,7 +21,11 @@ export interface CodeBlockProps {
 
 export function CodeBlock({ code, lang, className }: CodeBlockProps) {
   const trimmed = code.endsWith('\n') ? code.slice(0, -1) : code
-  const html = useMemo(() => highlightToHtml(trimmed, lang), [trimmed, lang])
+  // Re-render when a lazy grammar finishes loading, so a fence that showed plain
+  // text while its language's grammar imported picks up highlighting. The
+  // snapshot value is opaque; only its change across renders drives the memo.
+  const loaded = useSyncExternalStore(subscribeGrammarLoaded, grammarLoadCount, grammarLoadCount)
+  const html = useMemo(() => highlightToHtml(trimmed, lang), [trimmed, lang, loaded])
   const rootRef = useRef<HTMLDivElement>(null)
   const [copied, setCopied] = useState(false)