fix: reject malformed generated roots

README.md

@@ -209,8 +209,11 @@ pnpm install
 # Missing dependencies exit 42; stale/foreign apps/web/.next state exits 43.
 # The web build certifies its exact standalone symlink manifest; added, removed,
 # retargeted, or manifest-only-tampered generated links also exit 43. This detects
-# accidental/independent drift, not a same-UID actor that can rewrite both records
-# consistently (CWE-345); an external trust anchor is required for that boundary.
+# accidental, independent, stale, and foreign-residue mutation—the class exposed by
+# a five-month-stale .next that produced 19 phantom TS2307 errors.
+# It does NOT defend against a same-UID actor that can rewrite both manifest and
+# marker consistently (CWE-345). RM-59 tracks the required executor/spine-side
+# trust anchor outside worktree authority.
 pnpm preflight
 
 # Optional local queue service only. This does not start PostgreSQL.

docs/scratchpads/rm-01-reproducible-checkout.md

@@ -46,7 +46,7 @@
 - The original blanket symlink wording conflicts with AC4 because canonical Next `output: 'standalone'` emits legitimate pnpm dependency symlinks. The coordinator independently verified 42 such links and approved the operative restatement: `.next` itself must not be a symlink; descendant symlinks must exactly match the successful build's certified manifest.
 - RED-first controls were observed failing together against the prior implementation (exit 1): `.next` root, added, removed, retargeted, tampered-manifest, and canonical-style certified-link cases. The build now publishes the manifest atomically before the existing source certification commit marker; that marker binds the manifest SHA-256. Missing/partial/modified manifests remain untrusted.
 - GREEN evidence: the six-case symlink control passes; the exact reviewer-added link exits 43; removing it restores preflight exit 0. The added RED-first build-publication control also proves a symlinked `.next` cannot redirect certification writes outside the checkout. `pnpm test:checkout` passes 23 top-level tests / 29 including subtests. Canonical `pnpm --filter @mosaicstack/web build` and the following `pnpm preflight` both exit 0.
-- Threat-model ruling: the manifest detects accidental, independent, and stale mutation only. It does not defend against a same-UID actor able to rewrite both manifest and marker consistently (CWE-345); no local worktree construction can without an external trust anchor. The residual belongs to the planned choke-point executor / PostgreSQL spine where verification can occur outside worktree authority.
+- Threat-model ruling: the manifest detects accidental, independent, stale, and foreign-residue mutation—the class exposed by the five-month-stale `.next` that produced 19 phantom TS2307 errors. It does not defend against a same-UID actor able to rewrite both manifest and marker consistently (CWE-345); no local worktree construction can without an external trust anchor. RM-59 tracks the residual: executor/spine-side attestation outside worktree authority, dependent on RM-12, RM-21, and RM-25.
 - AC8 concrete proof at `df7530ae`: a clean clone ran in `ci-base:latest` with Docker `--read-only`; its only writable mounts were `/workspace` (the worktree) and `/home/ci` (`HOME`, with `NPM_CONFIG_STORE_DIR=/home/ci/store`). `pnpm install --frozen-lockfile && pnpm -w typecheck` exited 0 with 45/45 uncached tasks. This proves the implemented checkout path requires no writable location outside `$HOME` and the worktree. An initial fixture attempt failed only because Git required `/workspace` safe-directory setup; it is not counted as evidence.
 
 ## Handoff

scripts/preflight.mjs

@@ -164,11 +164,11 @@ export async function runPreflight({ root = process.cwd(), uid = process.getuid?
   let generated = [];
   try {
     const nextStats = await lstat(nextDir);
-    if (nextStats.isSymbolicLink()) {
+    if (!nextStats.isDirectory() || nextStats.isSymbolicLink()) {
       return {
         code: GENERATED_STATE_EXIT,
         message:
-          'MOSAIC_PREFLIGHT_GENERATED_STATE: apps/web/.next contains a symbolic link and is not trustworthy; run pnpm clean:generated, then rerun the gate',
+          'MOSAIC_PREFLIGHT_GENERATED_STATE: apps/web/.next must be a real directory, not a symbolic link, and is not trustworthy; run pnpm clean:generated, then rerun the gate',
       };
     }
     generated = [nextDir, ...(await entries(nextDir))];
@@ -183,11 +183,15 @@ export async function runPreflight({ root = process.cwd(), uid = process.getuid?
       if (uid !== undefined && stats.uid !== uid) foreign.push(path.relative(root, target));
     }
 
-    // Threat model: this detects accidental, independent, or stale generated-state
-    // mutation. It does NOT defend against an actor with same-UID write access to
-    // the generated tree: that actor can regenerate both the manifest and marker
-    // consistently (CWE-345). No local construction can without a trust anchor
-    // outside that actor's authority.
+    // Detects accidental, independent, stale, and foreign-residue mutation of
+    // generated state: the class this check was born from was a five-month-stale
+    // .next whose validator referenced deleted pages and produced 19 phantom TS2307
+    // errors indistinguishable from real type errors.
+    //
+    // Does NOT defend against an actor with same-UID write access to the generated
+    // tree, which can regenerate both the manifest and marker consistently
+    // (CWE-345). No local construction can, absent a trust anchor outside that
+    // actor's authority. RM-59 tracks executor/spine-side attestation.
     let certification = null;
     let certifiedManifest = null;
     try {

scripts/preflight.test.mjs

@@ -128,6 +128,17 @@ test('generated-state symbolic links are accepted only when exactly build-certif
     assert.match(result.message, /symbolic link/);
   });
 
+  await t.test('apps/web/.next is rejected when it is not a directory', async () => {
+    const root = await fixture('non-directory-next-root');
+    await installRequiredBins(root);
+    await writeFile(path.join(root, 'apps', 'web', '.next'), 'not a Next build\n');
+
+    const result = await runPreflight({ root });
+    assert.equal(result.code, 43);
+    assert.match(result.message, /MOSAIC_PREFLIGHT_GENERATED_STATE/);
+    assert.match(result.message, /real directory/);
+  });
+
   await t.test('an added descendant symlink is rejected', async () => {
     const root = await fixture('symbolic-next-added');
     await installRequiredBins(root);