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.
Serhii Dziupin
69150366fd