Complete the Sequence Action path for running multiple transition activities

[sfmskywalker]

Objective

Make “Run multiple activities” a complete, understandable path from an empty THEN slot to editing a Sequence and its children.

Context

• PRD: PRD: WF4-aligned StateMachine authoring and execution for Elsa 3.8 elsa-foundation#1456
• A transition has one Action activity; composition uses Elsa.Sequence.
• Existing Sequence designer and embedded branch root chooser provide reusable seams.

Scope

• Surface Sequence as a prominent picker option.
• Create it through the standard descriptor/identity path.
• Ensure Open reaches the Sequence editing experience.
• Verify child activity selection/update round-trips through the StateMachine slot.
• Add empty/read-only/unknown child handling where required by the existing nested designer.

Non-goals

• Adding an array-valued Action.
• Reworking the Sequence designer generally.
• General nested-activity navigation beyond this slot path.

Architecture constraints

Preserve one activity in the transition Action slot and existing activity resolver identity rules.

Dependencies

• Searchable picker and safe slot controls.
• Existing Sequence designer contract.

Affected repositories

• elsa-workflows/elsa-studio

Acceptance criteria

• “Run multiple activities” selects Elsa.Sequence.
• The created Sequence has valid activity and node identity.
• Open exposes the Sequence's child authoring experience.
• Child edits update the owning transition Action only.
• Save/reload preserves Sequence and child identity.
• Read-only allows inspection without mutation.

Validation

Component/session tests plus a manual or automated nested Sequence authoring demonstration.

Agent readiness

Not Ready — depends on the picker task and verification of the nested selection path.

[sfmskywalker]

Dependency gate is satisfied by 402168f1; implementation is now claimed. Scope remains the standard Elsa.Sequence path, nested child selection/update ownership, save/reload identity, and read-only inspection.

[sfmskywalker]

Implemented in 69285b09. “Run multiple activities” creates a normal Elsa.Sequence; Open now enters its established designer even in read-only mode. Nested child selection and cloned updates round-trip through only the owning Action slot while preserving StateMachine/Sequence/child identity across export and reload. Validation: Workflows 105/105; Designer 65/65 on net8/net9/net10.