Highlight code while it streams, so settling doesn't pop

[milind-soni]

In plain terms

After a streamed reply with code finished, its code blocks rendered plain for a beat and then "popped" into color — the last visible rough edge at stream end (follow-up to #202).

Why

While streaming, CodeBlock skipped shiki entirely and never wrote the highlight cache. The settled bubble is a fresh component instance, so its blocks mounted plain, then async shiki swapped highlighted HTML in.

The fix

Highlight during streaming, debounced: a block whose content has held still for 250ms (the fence is very likely complete) tokenizes and caches. The debounce is the effect lifecycle itself — every content change re-runs the effect and the cleanup clears the pending timer, so a growing block never tokenizes per token. The settled bubble then mounts straight from cache: no pop. Partial fences may cache under their own content hash — harmless (distinct keys, evicted by the existing cap), and the header comments now state the new invariant honestly.

Non-streaming behavior is unchanged.

Test plan

• pnpm typecheck clean; full pnpm vitest run green (736 passed)
• oxlint adds no findings over base (the one hit is pre-existing, verified via stash)
• Reviewer eyeball: stream a reply containing a fenced code block — it should colorize ~250ms after the block stops growing, and stay stable through the end of the stream

🤖 Generated with Claude Code

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