perf: cut the cost of Jest's file indexing

.nx/version-plans/version-plan-1792138223000.md

@@ -0,0 +1,5 @@
+---
+__default__: patch
+---
+
+Harness runs use less CPU. Jest now builds its file map in a single process instead of starting a pool of worker processes on every cold start or file change, and it no longer indexes Harness's `.harness` cache or iOS and Android build output (`ios/Pods`, `ios/build`, `android/build`, `android/.gradle`, `android/app/build`, `android/app/.cxx`). Jest also finds files without watchman, which is now faster than querying it; Metro still uses watchman.

packages/jest/src/jest-preset.ts

@@ -1,3 +1,21 @@
 export default {
   runner: '@react-native-harness/jest',
+  // Tests run serially on the device, so Jest only uses its workers to build
+  // the haste map. On a cold cache or after any file change that starts a pool
+  // of `availableParallelism() - 1` processes, which roughly doubles Jest's
+  // CPU time without making the run faster.
+  maxWorkers: 1,
+  // Build and cache output never contains tests, but it can hold tens of
+  // thousands of files. Jest checks these patterns before descending into a
+  // directory, so its crawl skips them entirely. Jest appends a project's own
+  // patterns to the preset's.
+  modulePathIgnorePatterns: [
+    '/\\.harness(/|$)',
+    '/ios/(Pods|build)(/|$)',
+    '/android/(build|\\.gradle|app/build|app/\\.cxx)(/|$)',
+  ],
+  // With the build output skipped, Jest's own crawl is cheaper than querying
+  // watchman, which costs two CLI process spawns per run and still lists every
+  // file. Metro keeps using watchman.
+  watchman: false,
 };