fix: simplify 'as unknown as X' casts to 'as X' where compiler allows

[KuraPiee]

Closes #5

Summary

Performs a mechanical cleanup across the codebase, simplifying \�s unknown as X\ assertions down to plain \�s X\ assertions where TypeScript single-step assertion rules allow.

• Tested every double-hop cast across the repository against \ sc --noEmit.
• Replaced 30 instances across 13 files where target types sufficiently overlap the source types, removing unnecessary intermediate \unknown\ hops.
• Preserved existing comments and left double casts intact only where TypeScript strictly requires them.
• All checks (\pnpm typecheck, \pnpm lint, \pnpm build) pass cleanly.