dist/ contents don't match package.json exports map — consumers hit 404 on ESM import

[loadfix]

Problem

This breaks downstream consumers today. The published package (xlsx-preview@0.0.2) declares in `package.json`:

```json
"types": "dist/xlsx-preview.d.ts",
"exports": {
".": {
"import": "./dist/xlsx-preview.mjs",
"require": "./dist/xlsx-preview.js",
"types": "./dist/xlsx-preview.d.ts"
}
}
```

But `dist/` contains only:

```
xlsx-preview.js (UMD)
xlsx-preview.js.map
```

Missing: `xlsx-preview.mjs`, `xlsx-preview.min.js`, `xlsx-preview.min.mjs`, `xlsx-preview.d.ts`.

Repro

• Bundler consumer doing `import { parseAsync } from 'xlsx-preview'`: module resolution walks `exports.import` to `./dist/xlsx-preview.mjs` → ENOENT.
• TypeScript consumer: looks for `./dist/xlsx-preview.d.ts` → ENOENT, types fall back to `any`.
• Only `require('xlsx-preview')` (CommonJS / UMD) currently works as advertised.

Root cause

`rollup.config.mjs` lines 29-63 only emit `.mjs` / `.d.ts` / `.min.*` under `BUILD=production`. The default `npm run build` emits only UMD. The committed `dist/` reflects a non-production build.

Two fix options

Option 1 (recommended): Update `scripts.build` to invoke `build-prod` equivalent so the committed `dist/` matches the advertised surface. Verify all five artifacts are produced and committed.

Option 2: Narrow `package.json` `exports` to the two files that actually exist, dropping the `.mjs` / `.d.ts` advertisements until build is fixed.

Option 1 is better — ESM + types are what consumers expect from a modern TS library.

Blocker for

• npm publish (would ship a broken package as-is)
• any downstream project migrating docxjs → xlsxjs that relies on the same import shape

Surfaced by: 2026-05-05 consistency audit. Previously noted in `TODO.md` § "Audit findings 2026-05-05" item 1.