fix(snapshots): preserve registry overlays in direct mode

Author: fengmk2

crates/vp_cli_snapshots/tests/cli_snapshots/README.md

@@ -170,9 +170,11 @@ response cache. `VP_SNAP_REGISTRY_MODE=direct` restores the original routing for
 an independent wall-time baseline, but cannot log traffic that bypasses the
 local registry. Keep builds unchanged and use fresh response directories.
 Local tarball URLs can cause snapshot differences in comparison modes.
-Locally authored packages remain local in both comparison modes because no
-published version exists. Cases that use them must set `local-registry = true`
-so `direct` mode also starts their registry.
+Fixture overlays remain active in both comparison modes. Cases with a
+`mock-manifest.json` or `tarballs/` start a private registry even in `direct`
+mode, so fixed templates remain the same test inputs. Cases that use shared
+locally authored packages must set `local-registry = true` so `direct` mode
+also starts their registry; these packages have no published version.
 
 With `VP_SNAP_ARTIFACTS_DIR` set, each runner process creates a unique `run-*`
 directory. `runner/timing.json` records discovery, test execution, and final

crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/local_registry/assert-overlay.mjs

@@ -0,0 +1,20 @@
+import assert from 'node:assert/strict';
+import { createHash } from 'node:crypto';
+
+const registry = process.env.NPM_CONFIG_REGISTRY;
+assert.ok(registry, 'Fixture overlay registry was not started');
+assert.equal(new URL(registry).hostname, '127.0.0.1');
+
+const metadataResponse = await fetch(`${registry}/@vp-smoke/hello`);
+assert.equal(metadataResponse.status, 200);
+const metadata = await metadataResponse.json();
+assert.equal(metadata.name, '@vp-smoke/hello');
+assert.equal(metadata['dist-tags'].latest, '1.0.0');
+
+const { dist } = metadata.versions['1.0.0'];
+assert.equal(new URL(dist.tarball).origin, new URL(registry).origin);
+const tarballResponse = await fetch(dist.tarball);
+assert.equal(tarballResponse.status, 200);
+const tarball = Buffer.from(await tarballResponse.arrayBuffer());
+assert.equal(`sha512-${createHash('sha512').update(tarball).digest('base64')}`, dist.integrity);
+console.log('Fixture metadata and tarball integrity verified');

crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/local_registry/snapshots.toml

@@ -7,3 +7,10 @@ steps = [
   { argv = ["npm", "install", "@vp-smoke/hello", "--no-save", "--no-audit", "--no-fund"], comment = "install a package served only by the local registry", snapshot = false, continue-on-failure = true },
   { argv = ["vpt", "print-file", "node_modules/@vp-smoke/hello/package.json"], comment = "the local registry served the packument and its tarball (integrity verified by npm)", continue-on-failure = true },
 ]
+
+[[case]]
+name = "local_registry_overlay"
+vp = "global"
+# No local-registry flag: fixture overlays must also work in direct mode
+# without opting into packed checkout packages.
+steps = [["node", "assert-overlay.mjs"]]

crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/local_registry/snapshots/local_registry_overlay.md

@@ -0,0 +1,7 @@
+# local_registry_overlay
+
+## `node assert-overlay.mjs`
+
+```
+Fixture metadata and tarball integrity verified
+```

crates/vp_cli_snapshots/tests/cli_snapshots/main.rs

@@ -1302,8 +1302,8 @@ fn run_case(
         case_env.get("PATH").cloned().unwrap_or_else(|| case_install.path_env.clone());
 
     // Cases without overlays share the HTTP server, but keep package-manager
-    // caches isolated. Fixture overlays retain a per-case server. Neither
-    // registry environment changes PATH.
+    // caches isolated. Fixture overlays retain a per-case server in every
+    // mode, including direct mode. Neither registry environment changes PATH.
     let registry_phase = report.phase("registry-startup");
     let mut shared_bun_cache = None;
     let (registry_env, registry) = if let Some(env) = shared_registry_env {
@@ -1330,6 +1330,8 @@ fn run_case(
         shared_bun_cache = Some(cache);
         (env, None)
     } else if case.local_registry
+        || stage.join("mock-manifest.json").exists()
+        || stage.join("tarballs").exists()
         || std::env::var("VP_SNAP_REGISTRY_MODE").as_deref() != Ok("direct")
     {
         let cache_dir = std::env::var_os("VP_SNAP_REGISTRY_CACHE_DIR")