fix(ui): correct markdown cache identity, streaming churn, and redundant tiers

Review follow-up on the #2769 highlight caches.

Fingerprint strength. The block/highlight caches are now global and
content-addressed, so a hash collision no longer mis-colors a block — it returns
a *different* block's rendered HTML and shows the user source they never wrote.
Length + one 32-bit FNV-1a is not enough key space for that failure mode at
session scale. `contentFingerprint` now combines two independent 32-bit
multiplicative hashes with a final avalanche (~64 bits); two multiplies per
character are free next to Shiki tokenization.

Streaming churn. Content addressing made every streaming step of the trailing
`live` block insert a new cache entry, so one long message evicted the settled
`full` blocks the fix exists to keep warm. `full` and `live` blocks now use
separate caches; the live cache is small (32 entries / 2MB) because it only has
to absorb repeat renders of the same step.

Redundant worker-side caches. `markdown-worker.ts` is the only sender to the
Shiki worker, and its client cache is larger than the worker-side ones, so the
worker caches could not serve a hit the client had not already served — they
only duplicated up to 48MB of payloads in a second heap. Removed; the reason
memoization belongs on the client is now documented there, along with why only
`highlightTokens` carries a theme in its key.

Dead `cacheKey` plumbing. `renderMarkdownBlocks` kept a `cacheKey` parameter it
only `void`-ed. Removed it and the now-unused `useMorphdomMarkdown` prop; the
remaining call-site local is renamed `fadeKey` for what it actually keys.

Tests: image-mode cache identity, streaming-does-not-evict-settled-blocks,
live-cache reuse, and a 20k same-length-source fingerprint collision check.
Each new guard was verified to fail without its fix.

packages/ui/src/components/chat/MarkdownRendererImpl.tsx

@@ -835,7 +835,6 @@ const useMorphdomMarkdown = ({
   containerRef,
   text,
   streaming,
-  cacheKey,
   imageMode = 'inline',
   syntaxVars,
   ctx,
@@ -843,7 +842,6 @@ const useMorphdomMarkdown = ({
   containerRef: React.RefObject<HTMLDivElement | null>;
   text: string;
   streaming: boolean;
-  cacheKey: string;
   imageMode?: MarkdownImageMode;
   syntaxVars: Record<string, string>;
   ctx: DecorateContext;
@@ -908,7 +906,7 @@ const useMorphdomMarkdown = ({
     const target = container.querySelector<HTMLElement>('[data-markdown-content]') ?? container;
     let active = true;
 
-    void renderMarkdownBlocks(text, streaming, cacheKey, imageMode).then((blocks) => {
+    void renderMarkdownBlocks(text, streaming, imageMode).then((blocks) => {
       if (!active) return;
       const existing = Array.from(target.children) as HTMLElement[];
 
@@ -959,7 +957,7 @@ const useMorphdomMarkdown = ({
     return () => {
       active = false;
     };
-  }, [containerRef, text, streaming, cacheKey, imageMode, ctx, refreshMermaidViewers]);
+  }, [containerRef, text, streaming, imageMode, ctx, refreshMermaidViewers]);
 
   React.useEffect(() => {
     const container = containerRef.current;
@@ -1040,13 +1038,13 @@ const MarkdownRendererImpl: React.FC<MarkdownRendererProps> = ({
 
   const syntaxVars = React.useMemo(() => getMarkdownSyntaxVars(currentTheme), [currentTheme]);
   const ctx = useDecorateContext(currentTheme, live, effectiveDirectory ? handlePreviewLoopback : undefined, DEFAULT_MERMAID_CONTROLS);
-  const cacheKey = `markdown-${part?.id ? `part-${part.id}` : `message-${messageId}`}`;
+  // Identity for the fade-in wrapper: a new part/message restarts the animation.
+  const fadeKey = `markdown-${part?.id ? `part-${part.id}` : `message-${messageId}`}`;
 
   useMorphdomMarkdown({
     containerRef,
     text: content,
     streaming: live,
-    cacheKey,
     imageMode: variant === 'assistant' ? 'label' : 'inline',
     syntaxVars,
     ctx,
@@ -1060,7 +1058,7 @@ const MarkdownRendererImpl: React.FC<MarkdownRendererProps> = ({
 
   if (isAnimated) {
     return (
-      <FadeInOnReveal key={cacheKey} skipAnimation={skipFadeIn}>
+      <FadeInOnReveal key={fadeKey} skipAnimation={skipFadeIn}>
         {markdownContent}
       </FadeInOnReveal>
     );
@@ -1137,9 +1135,6 @@ const SimpleMarkdownRendererImpl: React.FC<{
     containerRef,
     text: renderedContent,
     streaming: false,
-    // Identity is unused for cache lookup (content-addressed in markdownCore);
-    // keep a stable per-variant key for effect deps alongside `text`.
-    cacheKey: `simple:${variant}`,
     syntaxVars,
     ctx,
   });