Clean up the Application package's DI surface

[Vulthil]

Summary

• Remove the parameterless AddApplication()/AddHandlers() (guaranteed-throw) and AddFluentValidation()
  (silent no-op) overloads outright — maintainer-approved given no external consumer of this package exists
  yet — instead of obsoleting them; fix the "atleast" typo in the shared exception text on the surviving
  AddHandlers(HandlerOptions).
• PublicAPI.Shipped.txt is untouched; PublicAPI.Unshipped.txt carries the three *REMOVED* markers, and a
  new CompatibilitySuppressions.xml suppresses the resulting CP0002 findings against the 1.1.0 baseline.
• Delete dead internal plumbing: IRequestHandlerBase's untyped HandleAsync(object, ...) wrapper and
  NotificationHandlerExecutor.HandlerInstance (written, never read).
• Hoist per-dispatch allocations: PipelineHandlerDecorator materializes its reversed behavior chain once
  per scoped instance; the domain-event wrapper drops the same LINQ overhead where correctness allows;
  RequestLoggingPipelineBehavior/DomainEventLoggingPipelineBehavior hoist typeof(...).Name to static
  readonly fields.
• Cache FluentValidationAssemblies/HandlerAssemblies instead of reallocating a read-only copy on every
  access; document the two-level registration model — AddApplication(Action<ApplicationOptions>) is the
  host's composition entry point, while AddHandlers(Action<HandlerOptions>) and
  AddFluentValidation(Action<FluentValidationOptions>) are supported modular entry points a project that
  doesn't own the host can call directly and additively — in XML remarks and a new docs/articles/cqrs-pipeline.md
  subsection; add the missing = default to IDomainEventPublisher.PublishAsync's CancellationToken
  parameters (PublicAPI.Shipped.txt text-only edit).

Verification

• Full solution build (56 projects, incl. samples): clean, 0 warnings/errors — no in-repo caller
  referenced the removed members.
• Vulthil.SharedKernel.Application.Tests: 29/29 passed on net9.0 and net10.0, including a new test proving
  AddApplication + a second module's AddHandlers/AddFluentValidation calls compose additively without
  duplicating the shared ISender/IDomainEventPublisher registrations.
• Clean dotnet pack -c Release (fresh obj/bin) for Vulthil.SharedKernel.Application: succeeded with
  the CompatibilitySuppressions.xml in place; reproducibly failed with 6 CP0002 errors without it.
• PublicAPI.Shipped.txt unchanged except the two PublishAsync lines gaining
  = default(System.Threading.CancellationToken); the three removed members are marked in
  PublicAPI.Unshipped.txt instead.

Backport to v1.0: no — removals and guidance ship with the next minor.