feat(spec): add action duration metadata to manifest (Issue ebu#26)

[Richardpwe]

Implements the manifest-based action duration metadata discussed in Issue #26.

This PR adds optional actionDurations support to the Graphics manifest, schema, TypeScript definitions, and examples. Durations are expressed in milliseconds, with -1 indicating a dynamic or unknown duration. playAction can also define step-specific durations with an optional fallback entry.

Notes

The implementation differs slightly from the example shape discussed in the issue:

• It uses type instead of id for the action kind, matching the existing setActionsSchedule() action shape.
• It uses customActionId instead of name, because customActions[].name is already the display name, while customActions[].id is the technical identifier used by customAction().
• Step-specific playAction durations use zero-based step numbers, matching the existing playAction.goto semantics.

Closes #26.