build: stage beside dist/, not inside it

The staging directory was dist/.build-XXXXXX. A build that is killed cannot remove it, the next build removed
only its own, and "files": ["dist"] put the leftover into the npm tarball. .gitignore hid it from git status and
the freshness test skipped every name starting with ".build-", so nothing reported it.

The staging directory is now .dist-staging beside the output (.<name>-staging for --out <dir>) and is cleared at
the start of each build. The freshness test no longer skips anything: a stray file under dist/ fails it.

Checked in a scratch copy: a build stopped with SIGKILL after 1.5 s leaves .dist-staging/ and an unchanged dist/;
npm pack lists 36 files, none from the staging directory; the next build removes it. New test for the same on a
temporary package. npm test: 28 pass in 8.2-9.5 s.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/packaging.test.js

@@ -13,10 +13,9 @@ const pkg = require("alex2node/package.json");
 const ROOT = path.join(__dirname, "..");
 const DIST = path.join(ROOT, "dist");
 
-// Every file under dir, relative to it, sorted. A staging directory left by a killed build is not part of dist/.
+// Every file under dir, relative to it, sorted.
 function files(dir, prefix = "") {
   return fs.readdirSync(path.join(dir, prefix), { withFileTypes: true })
-    .filter((entry) => !entry.name.startsWith(".build-"))
     .flatMap((entry) => (entry.isDirectory() ? files(dir, path.join(prefix, entry.name)) : [path.join(prefix, entry.name)]))
     .sort();
 }