test(cli): no test boots `serve` and asserts the four cluster-driver diagnosis branches (follow-up promised in #14042)

[os-steve]

Follow-up disclosed in #14042 (the #13330 fix), filed as promised there. This is a coverage gap the author disclosed, not a defect: the at-tier contract review on #14042 judged it non-blocking on merits, but required the promised follow-up to exist as an issue.

What is untested

No test boots serve and asserts the four cluster-driver diagnosis branches added by #14042 (packages/cli/src/commands/serve.ts, the cluster block in the boot method):

1. registered — driver loaded and visible in the registry: silent (no diagnosis printed);
2. loaded-but-invisible — the driver evaluated without error but the registry this boot reads does not hold it (a physical-copy split): warns, naming the split and the one-line config remedy;
3. not-resolvable — prints the undeclared / declared-unresolvable classification from createHostImporter that the old silent catch swallowed;
4. resolved-then-crashed — the driver package's own evaluation error is printed instead of being reported as "not registered".

There is also a fifth, deliberately silent case — an app on an older @objectstack/service-cluster with no listClusterDrivers accessor prints nothing (declines to claim either answer) — which is equally unasserted at boot level.

Why it is untested

The reading sits inline in a very long boot method, so there is no seam to drive the branches through without booting the whole of serve.

Compensating coverage (why this is p3, not a defect)

• the serve-cluster-host-resolution source scan pins the shape of the block (32 tests);
• the CLI typecheck covers the block's types;
• the accessor invariant the reading rests on — listClusterDrivers() agreeing with defineCluster() — is pinned in both directions in packages/services/service-cluster/src/cluster-driver-registry.test.ts (suite 70/70 at fix(types,cli): resolve host-declared packages through the import condition, and read the cluster registry instead of assuming it #14042 head);
• every branch is print-and-continue: downstream behaviour was verified unchanged in the fix(types,cli): resolve host-declared packages through the import condition, and read the cluster registry instead of assuming it #14042 review (an absent driver still reaches defineCluster()'s documented error).

The risk the gap leaves open is the one the review named: silent-print branches are the exact shape that regresses invisibly.

Done means

A test (or tests) that boots serve (or extracts the reading into a testable seam first) and asserts each diagnosis branch fires on its condition — including that the registered case and the older-service-cluster case stay silent.

Refs: #14042, #13330.