Add startup-time validator for missing peer module dependencies

[antosubash]

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":

Missing peer	Symptom
Admin needs OpenIddict	Startup crash, Body was inferred… (misleading minimal-API error)
Email needs BackgroundJobs	Startup crash, Unable to resolve IBackgroundJobs
Admin/Users/etc. need Localization	Silent — every label renders as bare key (Users.Title, Roles.ColName)
Tenants needs FeatureFlags	Same *.Contracts-only ProjectReference shape

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.

[antosubash]

Resolved by #140.

framework/SimpleModule.Hosting/ModuleGraphValidator.cs runs as an IHostedService on startup, gated by SimpleModuleOptions.ValidateModuleGraph (default true). It scans loaded SimpleModule.*.Contracts assemblies for I*Contracts interfaces and logs a warning for each one that has no registered implementation, naming the missing module package. Skipped in Production to avoid false positives where a downstream app legitimately ships its own implementation of a contract.

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