Replace build tooling with tsdown for dual CJS/ESM output

[ikrbasak]

Problem

The current build pipeline uses tsc directly, which has several issues:

1. No ESM output

tsc emits only CommonJS. Modern bundlers (webpack, rollup, esbuild, vite) and Node.js
with "type": "module" can't tree-shake or properly consume the package without an ESM
build. There is no exports field or module field, so bundlers fall back to the CJS
bundle and lose the ability to do dead-code elimination.

2. Broken type declarations

package.json had "typings": "index.d.ts" pointing to a root-level file that doesn't
exist in the published package — the actual declarations were emitted to dist/.
TypeScript consumers would get no types unless they happened to find them via other means.

3. Tests import internal build artifacts

test/index.js imported validateUUID directly from ../dist/validate, coupling tests
to the internal module structure of the build output. This breaks whenever the bundler
changes how it splits or names output chunks.

4. No files field

Without it, npm publish includes source files, TypeScript sources, config files, and
other noise in the published tarball.

---

Fix

Replaces tsc with tsdown (built on rolldown) as the
bundler:

• Emits both CJS (dist/index.js) and ESM (dist/index.mjs) builds from the
  same TypeScript source
• Generates type declarations for both formats (dist/index.d.ts / dist/index.d.mts)
• Adds a proper exports map so Node.js and bundlers resolve the right format
  automatically
• Adds "module" field for bundlers that use it (webpack 4, rollup, etc.)
• Fixes "typings" → "types" pointing to the correct generated file in dist/
• Separates type-checking (tsc --noEmit as prebuild) from bundling, so the build step
  stays fast while type errors are still caught
• Adds a files field to keep the published package clean
• Inlines validateUUID into the test file with a regex, removing the dependency on
  internal build artifact paths

---

Why tsdown

tsdown is a TypeScript-first bundler built on rolldown (Rust-based), designed specifically
for library publishing. It handles the CJS/ESM dual-build, declaration generation, and
exports field wiring with minimal config compared to alternatives like tsup or a
hand-rolled rollup setup.

---

Changes at a glance

File	What changed
package.json	tsc → tsdown; added exports, module, types, files; fixed typings
tsdown.config.mjs	New bundler config: CJS + ESM output, dts, inline sourcemaps, minification
test/index.js	Removed ../dist/validate import; inlined UUID regex; guarded constants.default
package-lock.json	Lockfile updated with tsdown and its dependencies