test: split the packaging tier and parameterize its installed binary (task 0036)

The packaging tier held two kinds of assertion joined only by an
expensive shared setup: the shape of the packed tarball and its
manifest, and the behaviour of the resulting installed binary. Split
them, and teach the second to take the binary it drives from
GITEA_AXI_INSTALLED_BIN.

When that variable names an existing binary the tier drives it and skips
pack-and-install entirely; unset, it packs and installs exactly as
before. The installed-binary facet becomes one seam with two callers —
the npm distribution path today, the Nix installation path in task 0038
— so the two cannot drift apart in what they guarantee about an
installed gitea-axi. Nothing in it may assert on how the binary came to
exist, since store paths, wrapper internals, and the arrangement of the
installed tree are implementation detail of the installation method.

The tarball assertions stay npm-only: no other distribution method
produces a tarball or a packed manifest.

test/packaging/npm-artifact.ts

@@ -0,0 +1,53 @@
+import { execFileSync } from "node:child_process";
+import { mkdirSync } from "node:fs";
+import { join } from "node:path";
+import { fileURLToPath } from "node:url";
+
+// Building the npm distribution artifact, factored out so the two packaging
+// facets can each take just the part they need: the tarball facet packs and
+// extracts, the installed-binary facet packs and installs.
+
+export const projectRoot = fileURLToPath(new URL("../..", import.meta.url));
+
+/**
+ * Pack the real tarball into `destDir`, returning its path.
+ *
+ * `npm pack --json` reports the tarball filename; the shape shifted across npm
+ * majors (an array of entries on npm <12, an object keyed by package name on
+ * npm 12+), so accept either and pull the one filename out.
+ */
+export function packTarball(destDir: string): string {
+  const packOutput = execFileSync("npm", ["pack", "--json", "--pack-destination", destDir], {
+    cwd: projectRoot,
+    encoding: "utf8",
+  });
+  const packResult = JSON.parse(packOutput) as unknown;
+  const packEntries = Array.isArray(packResult)
+    ? (packResult as Array<{ filename: string }>)
+    : Object.values(packResult as Record<string, { filename: string }>);
+  return join(destDir, packEntries[0]!.filename);
+}
+
+/**
+ * Extract `tarball` into a fresh `extract/` under `destDir`, returning the
+ * directory npm nests everything under (`<destDir>/extract/package`).
+ */
+export function extractTarball(tarball: string, destDir: string): string {
+  const extractDir = join(destDir, "extract");
+  mkdirSync(extractDir);
+  execFileSync("tar", ["-xzf", tarball, "-C", extractDir]);
+  return join(extractDir, "package");
+}
+
+/**
+ * Install `tarball` globally into a throwaway prefix under `destDir`, returning
+ * the path of the installed binary on that prefix's bin dir.
+ */
+export function installGlobally(tarball: string, destDir: string): string {
+  const prefix = join(destDir, "prefix");
+  execFileSync("npm", ["install", "-g", "--prefix", prefix, tarball], {
+    cwd: projectRoot,
+    encoding: "utf8",
+  });
+  return join(prefix, "bin", "gitea-axi");
+}