Rename createState to createSignal

Written for AI agents. See Log Methodology Note below for details.

Why?

Because useState is the React style, not reactive state management, while createSignal is the solid.js reactive. Jay is using the latter, and therefore we should call the thingy by the accepted name, createSignal.

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Log Methodology Note

Note: These design logs are written primarily for AI agents as part of the Design Log methodology and made accessible here for human readers. The language and structure are optimized for machine consumption — expect precise, specification-style prose rather than narrative documentation.