build: migrate from unbuild to tsdown

[bytaesu]

Replaces unbuild with tsdown, bumps TypeScript to 6, and centralizes buffer coercion in a shared bytes helper

[Copilot]

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[greptile-apps]

Greptile Summary

This PR migrates the build tooling from unbuild to tsdown, bumps TypeScript from 5.8 to 6.0, and extracts shared buffer-coercion logic into a new src/bytes.ts helper (toBufferSource and toUint8Array) that all crypto/encoding modules now import.

• tsdown.config.mts mirrors the old build.config.ts entry list; unbundle: true keeps each entry as a separate emitted file so bytes.ts is naturally emitted as a shared chunk without needing an explicit entry.
• bytes.ts correctly handles the SharedArrayBuffer-backed view edge case (WebCrypto rejects SharedArrayBuffer), and its JSDoc documents the ArrayBuffer-vs-TypedArray memory semantics noted in a prior review thread.
• TypeScript 6 defaults types to [], no longer auto-including @types/node; password.node.ts uses Buffer as an ambient type that won't be in scope without \"types\": [\"node\"] in tsconfig.json.

Confidence Score: 4/5

Safe to merge once the tsconfig types omission is addressed; all other changes are mechanical refactors with no behavioral regressions.

The TypeScript 6 bump changes the default for types to an empty array, meaning @types/node ambient globals (specifically Buffer, used as a return-type annotation in password.node.ts) are no longer auto-included. This would make tsc --noEmit fail on the password.node.ts entry. Everything else looks correct.

Files Needing Attention: tsconfig.json needs types: [node] added to compilerOptions to keep @types/node ambient globals available under TypeScript 6.

_{Reviews (2): Last reviewed commit: "chore: add comment" | Re-trigger Greptile}