perf(tui): reuse streaming markdown instances

[liruifengv]

Related Issue

Resolve #990

Problem

See linked issue.

What changed

• Reuse the assistant Markdown component during streaming updates instead of recreating it on every flush.
• Add a transient Markdown theme mode that skips synchronous syntax highlighting while streaming, then render the final highlighted message once at stream end.
• Pass the transient state from the streaming controller and update tests for the new call signature.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: 83544f7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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[pkg-pr-new]

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commit: 83544f7