Engine-java package hygiene: move JSPromise to jsengine, document the package layout rule

[romain-pm]

Follow-up from the architecture review of the feature/js-server-extensions branch (PR #687). Internal-only refactor — no exported package is affected (Export-Package lists only …engine.sdk), so this is non-breaking and can land anytime.

1. Move JSPromise from …engine.actions to …engine.jsengine. It is a general GraalJS facility (settling promises through microtask draining), not an actions concern; its current home would force unrelated features (see Retrofit async (promise) support to server extension-point callbacks #688) to import from actions.
2. Document the package layout rule in a package-info.java for …engine.registrars: single-class bridges live flat in registrars/; multi-class verticals get a subpackage (validation/, contentpatches/); non-registrar surfaces live outside (actions/ endpoint, sdk/ facade). The rule exists implicitly today, which makes the mixed layout look accidental to newcomers.

Acceptance. JSPromiseTest moved along with the class; mvn test green; package-info.java present with the rule stated in 3–4 sentences.

[romain-pm]

Implemented on feature/js-server-extensions (commit 7c5be88): JSPromise (+tests) moved to ..engine.jsengine, package layout rule documented in registrars/package-info.java. Will be closed by the merge of #687.