fix: hide sourcemap references in published files

[yadokani389]

Summary

• Generate hidden sourcemaps with tsdown
• Keep sourcemaps available in local build output
• Stop emitting dangling sourceMappingURL annotations

Context

PR #1012 reduced the published package size by externalizing sourcemaps and excluding .map files from the npm package. However, generated JavaScript and declaration files still contained sourceMappingURL annotations pointing to those excluded files.

This caused consumers such as Vite to report missing sourcemap warnings.

Why

Using hidden sourcemaps preserves local debugging support while preventing consumers from requesting files that are not included in the npm package.

Testing

• pnpm run lint
• pnpm run typecheck
• pnpm test
• pnpm run build
• Verified that built files contain no sourceMappingURL annotations
• Verified that the npm package contains neither sourcemap files nor dangling sourcemap references