docs/emitter-design: target options are free-form, with no validation, versioning or lifecycle #706

Problem

EmitterInput.Options is "module path, root package, output layout — free-form per target". Free-form options can't be validated before Generate runs, can't evolve without breaking existing configurations, and give no way to change a default for new SDKs without changing it for existing ones.

Proposal

• Each target declares a typed options schema: required fields, defaults, validation, validated before Generate.
• The options carry a version, with migrations from each earlier version.
• Capabilities carries lifecycle: experimental, hidden, sunset.
• A behavior change that would alter existing SDKs is introduced as the default for new configurations only, pinned for existing ones through the manifest, with an upgrade report listing where a configuration differs from current defaults.