(cli): CDK Synth CLI command doesn't included stacks in nested cloud assemblies

### Describe the bug

When calling `cdk synth` on an app that has nested cloud assemblies (in this case, `Stage` subclass representing an application instance), the list of stacks printed after synthesis omits any stacks from the nested assembly.

This issue was described before, but closed when only part of the complaint was addressed: https://github.com/aws/aws-cdk/issues/18599

### Expected Behavior

I would expect all synthesized stacks to be listed

### Current Behavior

Only stacks that are direct children of the top level `App` are printed.

### Reproduction Steps

I've constructed a reproduction here: https://github.com/haljarrett/cdk-stage-issue/tree/master

In it, I have an App with two top level Stacks and a Stage, which itself has a Stack. Only the two top level stacks are printed when `cdk synth` is executed:

```
$ cdk synth

Successfully synthesized to /Users/haljar/src/stage-issue/cdk.out
Supply a stack id (toplevelA, toplevelB) to display its template.
```

Compare this to the correct output of `cdk list`:

```
$ cdk list

toplevelA
toplevelB
beta/nested
```

### Possible Solution

My thought is that this is arising from the way stacks are being selected from the root cloud assembly;

In [cdk-toolkit.ts->synth](https://github.com/aws/aws-cdk/blob/448e2bda5632621c057c2e448cd34cfec107b22e/packages/aws-cdk/lib/cdk-toolkit.ts#L645) we have the print statement generating this output, which uses stacks found from [selectStackForDiff](https://github.com/aws/aws-cdk/blob/448e2bda5632621c057c2e448cd34cfec107b22e/packages/aws-cdk/lib/cdk-toolkit.ts#L745).

This method pulls the stacks from the root cloud assembly with `assembly.selectStacks`, specifying the behavior as `MainAssembly` on [line 750](https://github.com/aws/aws-cdk/blob/448e2bda5632621c057c2e448cd34cfec107b22e/packages/aws-cdk/lib/cdk-toolkit.ts#L750C1-L750C1). My suggestion would be to move this behavior to `DefaultSelection.AllStacks`, which would pull stacks recursively from nested assemblies.

### Additional Information/Context

I am looking to move towards (and guide my team towards) using a CDK pattern that leverages `Stage` subclasses as the top level abstractionl; not being able to see / deploy the stacks that are built under a stage is a papercut when doing this.

### CDK CLI Version

2.114.1 (build 02bbb1d)

### Framework Version

_No response_

### Node.js Version

v20.6.0

### OS

MacOS

### Language

TypeScript

### Language Version

~5.2.2

### Other information

Noted in the above, but unless there is a strong reason to only use stacks from the main assembly in this output, this should be a trivial fix - maybe if I find some time over the weekend I'll review the contribution guidelines and cut a PR.