refactor: merge StageState into StageResult; per-stage error model

[dieppa]

Collapse StageState (was in flamingock-core) into a polymorphic class on StageResult
(in flamingock-core-commons), replacing the StageStatus enum. Per-stage errors now live
on StageResult.state and surface in the response without a pipeline-level fallback.

• StageState (polymorphic, Jackson @JsonTypeInfo): NotStarted, Started, Completed,
  Failed(ErrorInfo), BlockedForMI extends Failed (also carries List<RecoveryIssue>).
• StageResult.status: StageStatus → state: StageState. StageStatus enum removed.
• StageRun simplifies: single non-null StageResult, initialised with NOT_STARTED.
• ErrorInfo.changeId: String → changeIds: List<String>. fromThrowable signature updated.
• BlockedForMI synthesises its own ErrorInfo (type MANUAL_INTERVENTION_REQUIRED,
  changeIds populated). No more null-changeId leaking out.
• Runner's MI catch drops the redundant markPipelineFailed(miEx); BlockedForMI stages
  carry their own error info. findStageForChange throws IllegalStateException on an
  unmappable changeId (deterministic contract).
• RecoveryIssue gets @JsonCreator so BlockedForMI round-trips through Jackson.
• NOT_STARTED stages now appear in response.stages.

Wire format: StageResult.state is polymorphic ({"type":"FAILED","errorInfo":{...}}).
ErrorInfo.changeId renamed to changeIds (array).