Address PR review feedback (#1785)

- P2 (greptile): anchor the changed-test existence check at the git repo root
  (repoRootForExistence via `git rev-parse --show-toplevel`) instead of rootDir,
  so a script run from a package subdir without FUSION_PROJECT_DIR no longer
  forms a doubled path and silently drops live tests into the delegate path.
  +1 regression test (default root resolves to repo root).
- coderabbit: fix stale "1-worker lane" wording in the delegation log (now
  "heavy memory-envelope lane") and the "single-worker envelope" test title,
  both stale after the 1->4 worker change.

test-changed 118/118, eslint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/__tests__/test-changed.test.mjs

@@ -370,7 +370,7 @@ test("@fusion/core is a wide-fan-out memory-envelope package but is NOT gate-cov
   );
 });
 
-test("core scoped-affected env applies the bounded heap and single-worker envelope", () => {
+test("core scoped-affected env applies the bounded heap and worker envelope", () => {
   const env = createScopedAffectedMemoryEnvelopeEnv(CORE_SCOPED_AFFECTED_PACKAGE, {
     NODE_OPTIONS: "--trace-warnings",
     HOME: "/tmp/fusion-home",
@@ -1976,6 +1976,14 @@ test("existingChangedTestFilesInPackage: excludes non-test and out-of-package pa
   }
 });
 
+// FNXC:TestInfrastructure 2026-06-26-13:40: regression for the doubled-path subdir
+// bug — with NO projectRoot passed, the existence root must resolve to the git repo
+// root (not cwd), so a real repo-relative test path is found from any cwd.
+test("existingChangedTestFilesInPackage: default existence root anchors at the git repo root", () => {
+  const selfRel = "scripts/__tests__/test-changed.test.mjs"; // this very file — guaranteed on disk
+  assert.deepEqual(existingChangedTestFilesInPackage([selfRel], "scripts"), [selfRel]);
+});
+
 // FNXC:TestInfrastructure 2026-06-26-09:15: the merge gate re-covers a delegated
 // engine lane (curated engine-core subset) but runs NO dashboard tests. Lock that
 // asymmetry so the delegation messaging never overclaims dashboard gate coverage.

scripts/test-changed.mjs

@@ -1469,7 +1469,26 @@ handing Vitest an empty/garbage positional set.
  * @param {{ projectRoot?: string }} [opts]
  * @returns {string[]}
  */
-export function existingChangedTestFilesInPackage(changedFiles, pkgDir, { projectRoot = rootDir } = {}) {
+/*
+FNXC:TestInfrastructure 2026-06-26-13:40:
+Existence checks for changed test files must anchor at the GIT REPO ROOT, not
+`rootDir` (which falls back to `process.cwd()`). `git diff --name-only` returns
+repo-root-relative paths regardless of the cwd it runs from, so joining them
+against a `rootDir` that is a package SUBDIR (script run from a package without
+FUSION_PROJECT_DIR) would form a doubled path (packages/x/packages/x/...), make
+`existsSync` false, and silently DROP live changed tests into the delegate path.
+Resolve the toplevel once via git so the check is correct from any cwd inside the
+repo; fall back to rootDir if git can't report a toplevel.
+*/
+let _repoToplevelCache;
+function repoRootForExistence() {
+  if (_repoToplevelCache !== undefined) return _repoToplevelCache;
+  const top = gitOutput(["rev-parse", "--show-toplevel"]);
+  _repoToplevelCache = top && top.trim() ? top.trim() : rootDir;
+  return _repoToplevelCache;
+}
+
+export function existingChangedTestFilesInPackage(changedFiles, pkgDir, { projectRoot = repoRootForExistence() } = {}) {
   return (changedFiles ?? []).filter(
     (file) =>
       isTestFilePath(file) &&
@@ -1794,7 +1813,7 @@ export async function main(argv = process.argv.slice(2)) {
           const log = gateCovered ? console.log : console.warn;
           log(
             `[test-changed] ${pkg}: a changed non-test source file (${wideSourceDesc}) ` +
-              "would fan `vitest --changed` out to ~the full suite at this heavy 1-worker lane; " +
+              "would fan `vitest --changed` out to ~the full suite at this heavy memory-envelope lane; " +
               `no directly-changed ${pkg} test file to run, so ${delegationNote}`,
           );
           continue;