switch build to tsdown

[deepso7]

Summary

• replace the raw Rolldown build config with a tsdown config
• update package exports to tsdown's default .mjs / .d.mts output
• refresh the pnpm lockfile for the new build dependency

Verification

• pnpm run build
• pnpm test -- --run

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Summary by cubic

Switch build tooling from rolldown to tsdown to simplify the setup while keeping ESM and CJS builds, and update dev tooling. No runtime changes.

• Dependencies
  • Replace rolldown with tsdown; remove rolldown config and add tsdown config (ESM + CJS, DTS sourcemaps).
  • Update ESM outputs/exports to .mjs and .d.mts; CJS .cjs/.d.cts unchanged.
  • Simplify build script to tsdown.
  • Bump dev deps: @biomejs/biome, @effect/language-service, @types/node, tsx, ultracite, vitest.
  • Add changeset and refresh the pnpm lockfile.

^{Written for commit 4a9121c. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}