Document and verify the public API compatibility contract for 1.x

Problem/Motivation

The shared SuppressionPolicyInterface and SourceContext already provide an extension point, but stable adopters need an explicit statement of which interfaces, service aliases and structured outputs carry backward-compatibility commitments.

Proposed resolution

Document the existing public surface and mark implementation details internal. Review contracts before the first stable tag without redesigning working APIs or adding speculative abstractions.

Acceptance criteria

• Specify decide() inputs, SourceContext trust boundary, decision reasons, time basis, expiry and failure behavior, including disabled suppression.
• Identify supported service/interface aliases and the compatibility policy for Drush JSON output, reason codes and configuration.
• Document deprecation and migration expectations for future 1.x changes and distinguish public APIs from storage internals.
• Provide a small external-consumer example and meaningful contract tests for supported behavior.
• Confirm core-mail and Mailer Plus adapters obey the documented contract.

Scope and dependencies

Stable-release acceptance task under #3621224: Complete the 1.0.0 stable release verification gates. Builds on #3621126: Add a shared suppression decision service with a stable extension API; the completed original issue remains fixed.