Clarify host composition of dependency-owned DI preprocessors

[flancer64]

Context

The host Container Configurator is the only composition root for a TeqFW CLI application. Its container.preprocessors and container.postprocessors lists contain producer Dependency Identifiers. A package can publish one of those producers, but adding the package to dependencies or registering its namespace does not automatically add the producer to the host Container policy.

A host integration of TeqCMS exposed this gap: its web handler required a contract substitution, but the application had only configured its own preprocessor. The request resolved the placeholder API class and failed at runtime with Method not implemented.

Related closed issues #12 and #13 cover the preprocessor function type and the general host/package configurator boundary. This request is narrower: make the dependency-owned Container policy composition step explicit for consumers.

Requested documentation

Update the Host Container Configurator guide and a canonical example to explain that the host must explicitly include any dependency-owned preprocessor/postprocessor producer required by its selected composition. Show the producer Dependency Identifier in the host's ordered container.preprocessors list, or a compatible package Configurator contribution statically imported and merged by the host. State that the host must verify the contribution against the installed package contracts and choose deterministic ordering.

Acceptance criteria

• The guide explicitly says dependency installation, namespace discovery, and CLI plugin metadata do not automatically configure a package's Container policy producers.
• The example shows a host including a dependency-owned preprocessor producer alongside its own producer.
• The text preserves one host configurator and does not imply plugins can mutate the Container after creation.
• Verification guidance calls for resolving or exercising the contract whose substitution depends on that contribution.