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/markdown/markdown-worker.ts

@@ -18,6 +18,15 @@ import type { MarkdownTokenRun, MarkdownWorkerRequest, MarkdownWorkerResponse }
 // re-highlight load in openchamber/openchamber#2769. In-flight requests with
 // the same key coalesce so remount storms share one round-trip. Cache keys are
 // fingerprints (not full source) so large files are not duplicated in the Map.
+//
+// This module is the only sender to the worker, so memoizing here is sufficient
+// and the worker itself stays stateless apart from the Shiki instance. A second
+// cache inside the worker would only duplicate these payloads in another heap.
+//
+// `highlight` / `highlightLines` results are theme-independent: the worker
+// tokenizes with the CSS-variable `MARKDOWN_SHIKI_THEME`, so a theme switch
+// repaints via CSS and must not invalidate these entries. Only
+// `highlightTokens` resolves concrete colors, so only its key carries a theme.
 
 type PendingResolver = (response: MarkdownWorkerResponse | null) => void;