Phase 55 — withGroup preflight validator (consumer migration)

What changes. Client.boot now refuses to start when every consumer-supplied ClientModule in the deployment has Group = None (no |> ClientModule.withGroup "<name>" call anywhere in the module list). The validator runs once at startup alongside validateHandlers / validateRequestSeam and throws a clear failwithf naming the first ungrouped module.

Why. The sidebar's reserved _other bucket renders title-less and starts collapsed when no other declared group is present. A deployment whose every user module is ungrouped therefore boots with an empty-looking sidebar — even though every module is registered, accessible, and wired correctly. A single-module bring-up hit this; documentation alone proved insufficient.

Scope. Client-side only. ClientModule.Group is a client-tier concept with no server-side projection, so the check lives in src/ToolUp.Platform.Client/Client/ModuleGroupingValidator.fs and is invoked from Client.boot. No server-side IConfigValidator ships for this concern.

Who's affected

A consumer is affected if every module in its Client.run config modules list is constructed without |> ClientModule.withGroup "<name>". A single withGroup anywhere in the list satisfies the check. A multi-module deployment that already groups at least one module is N-A; the affected case is a single-module (or wholly ungrouped) deployment.

Diff to apply (single ungrouped module — representative)

// Before — single ungrouped module, sidebar collapses to title-less _other:
let register () : ErasedModule =
    ClientModule.create { Init = init; Update = update; Name = "Convert"; Icon = Icons.upload }
    |> ClientModule.withView (fun model dispatch -> view model dispatch)
    |> ClientModule.register

// After — assigns the module to a declared group:
let register () : ErasedModule =
    ClientModule.create { Init = init; Update = update; Name = "Convert"; Icon = Icons.upload }
    |> ClientModule.withView (fun model dispatch -> view model dispatch)
    |> ClientModule.withGroup "Workflow"
    |> ClientModule.register

Pick any group name that fits the deployment's information architecture — modules sharing a group name render together under a collapsible sidebar header. A single-module deployment can use anything ("Main", the app name, the workflow name); the name is just the section label.

Verification

1. dotnet build clean on the consumer side.
2. Boot the consumer; confirm the deployment reaches the shell render (the validator no longer throws at startup).
3. Confirm the sidebar renders the registered module under its declared group header — no empty-sidebar regression.

Rollback

The validator is the only change; reverting Client.boot's call site removes it. The consumer's withGroup call is otherwise harmless and should stay even after a rollback — it's the correct sidebar shape regardless of whether the validator is in place.

Consumers

The migration is N-A for any deployment that already groups at least one module; only single-module or wholly ungrouped deployments need the one-line withGroup addition.