Improve Decode with switch to using local buffers

[hloeung]

Split these changes out from #677

[Copilot]

Pull request overview

This PR refactors User.Decode() to avoid using u.BufferedMsg shared state by switching to a goroutine-local buffer and a buffered channel, and it changes ghost-user behavior to skip Decode() rather than blocking forever.

Changes:

• Replace the global u.BufferedMsg accumulation with a local bufferedMsg inside the decode buffer goroutine.
• Use a buffered buffer channel (size 512) and consolidate flush logic into a helper.
• For ghost users, return early from Decode() with a debug log instead of blocking.

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