test: assert every path the package manifest declares names a file - #1857

Problem

An export condition matches on key presence, not on whether its target resolves, so a target naming a missing file fails with ERR_MODULE_NOT_FOUND instead of falling through. ./metro's source target named ./src/metro.ts for a module authored .tsx until 361e90b. It was the third broken target in this manifest after #1638 (., ./babel) and #1723 (./types), and consumers found all three.

Why it isn't caught today

Jest, tsc and eslint never resolve the manifest, and .github/scripts/consumer_rc.py loads nativewind/metro and nativewind/babel under Node's default conditions, so no source or types target is ever resolved.

Solution

361e90b fixed ./metro, so this PR now carries only the guard. src/__tests__/package-exports.ts walks main, module, types and every leaf of the exports condition tree and asserts each names a file that exists. The list comes from the manifest, so a new export is covered as soon as it is added, and two cases assert the list is non-empty, so a manifest that stops being read fails instead of passing. Targets under bob's output directory are checked once a build has run, which CI does before testing and prepublishOnly before publishing.

Tests

With ./metro's source set back to ./src/metro.ts, the guard fails on that one target and the other three cases pass:

- Array []
+ Array [
+   "exports[\"./metro\"] [source] -> ./src/metro.ts",
+ ]

Verification

On Windows with Node 26: yarn lint clean · yarn typecheck clean · yarn build clean · yarn test --maxWorkers=2 --coverage 11 suites, 103 tests passed · yarn example expo export --platform web clean · no unstaged files after either build · release guards 17 passed. Not run: the iOS dev build, which no file in this change reaches.

Base

Cut from f9cfae6c, the current main.