Add startup-time validator for missing peer module dependencies #130

Across one consumer integration, four separate failure modes all came down to "module X is in the assembly graph but its peer module is missing":

The crash cases are partially addressed by declaring runtime deps as PackageReferences (separate issue). But the silent-fallback cases (Localization missing → bare keys; menu items pointing to nonexistent peer routes) need an actual diagnostic.

Suggested fix

A single boot-time pass that:

1. Scans [Module]-attributed types for declared dependencies
2. Inspects endpoint handlers' DI parameter requirements
3. Inspects shared useTranslation() calls vs TranslationLoader registration
4. Emits clear diagnostics:
   • ModuleX endpoint Y requires service Z which is not registered (install module Q)
   • ModuleX uses translations but SimpleModule.Localization is not installed; UI labels will render as bare keys

Could ship as builder.AddSimpleModule(o => o.ValidateModuleGraph = true) for opt-in until stable.