ci(test): download the Electron binary once before unit specs run (#1739)

docs/architecture/validation-map.md

@@ -107,6 +107,19 @@ with `diagnostics: false`); `isolatedModules`-incompatible syntax such as a
 type re-export without `export type` still fails at load time, and spec type
 errors are caught by `typecheck:spec` (see Unit And Type Checks).
 
+Some `electron-backend` and `database` specs run SQLite code in the Electron
+binary under `ELECTRON_RUN_AS_NODE`, resolving it with `require('electron')`.
+pnpm does not run Electron's postinstall (`electron` is deliberately absent from
+`onlyBuiltDependencies`, see [workspace shell](workspace-shell.md)), so the
+first `require` downloads and extracts the binary. With projects and Jest
+workers in parallel, one process can exec it while another is still extracting
+it (`spawnSync … ETXTBSY` on Linux). When a selected project has a spec that
+calls `createRequire(...)('electron')`, `coverage:unit:ci` first runs
+`tools/testing/ensure-electron-binary.mjs`, which downloads the binary once and
+fails unless it runs and reports the pinned version; CI also runs it as its own
+step when the Tier A suite is in scope. In a fresh worktree, run it before
+starting several of these specs at once by other means.
+
 In CI, a pull request skips the Tier A suite (and the merged-coverage upload)
 when every changed file is outside Tier A test inputs:
 `tools/coverage/unit-coverage-scope.mjs` holds the allowlist (Markdown, `docs/`,