ci: matrix over Node 22 and 24, add the bench and packaging tiers (task 0040)

The workflow pinned Node 20, which reached end-of-life in April 2026, while
the manifest promised support down to it — so the entire claimed range below
the single tested version went unverified and its floor was unsupported.

Matrix over the two supported long-term-support majors and narrow the engine
range to `^22 || ^24`, naming exactly what is tested. Narrowing is free now
because nothing has been published and no tags exist.

The benchmark harness tier joins every leg: it is deterministic and needs no
network, and its non-default runner configuration made it easy to believe it
was running when it was not. The end-to-end and packaging tiers run on the
highest leg only, conditioned on a flag attached to that leg through a matrix
`include` entry rather than on a version number restated at each site. The
benchmark smoke tier stays out, since it would pass by skipping.

`@types/node` follows the new floor; it was the last Node 20 reference in the
manifest, and the typecheck runs on every leg.

test/packaging/tarball.test.ts

@@ -63,8 +63,13 @@ describe("npm distribution artifact", () => {
 
     expect(packedManifest.license).toBe("MIT");
 
+    // The declared range names exactly the majors continuous integration
+    // matrixes over, so nothing is promised that is never tested. Node 20 is
+    // end-of-life and deliberately no longer named.
     const engines = packedManifest.engines as Record<string, string> | undefined;
-    expect(engines?.node).toMatch(/20/);
+    expect(engines?.node).toMatch(/22/);
+    expect(engines?.node).toMatch(/24/);
+    expect(engines?.node).not.toMatch(/20/);
 
     expect(packedManifest.type).toBe("module");
   });