@spec build_extensions(map(), map()) :: map()

Builds the projection extensions map.

When spec.projection_enricher names a module exporting enrich/2, it is called with the base extensions and the runtime, and its return replaces the extensions map. When absent, extensions contain just the runtime meta (if any).

This is shared so all profiles (stepwise, FSM, flow) can populate extensions consistently.

@spec existing_atom(String.t()) :: atom() | nil

Safe conversion of a binary key to an existing atom. Returns nil if the atom doesn't exist in the atom table, rather than raising.

@spec interpolate_topic(String.t(), map()) :: String.t() | nil

Replaces {key} placeholders in a topic template with values from the runtime context. Returns nil if any placeholder is missing or empty.

@spec subscriptions_for(map(), map()) :: [String.t()]

Builds the subscriptions list from spec declarations and runtime context, prepending the mandatory workflow_execution topic and interpolating {placeholder} variables from runtime context.

@spec ui_for(map(), atom() | String.t(), map()) :: map() | nil

Resolves a UI descriptor for a given state from the spec definition.

Looks up state_def.ui with optional key and assigns and merges in context assigns (context and state). Returns nil if no UI key is configured for the state.