Task: Let a definition select its step chain

A definition cannot choose what it runs: the chain is fixed in report_execution_workflow.hpp and the trigger hard-codes its type, so every definition gathers and assembles before it submits. Give the definition a carrier and register a shorter chain that starts from a bundle that already exists.

• The chain a definition selects is the one that runs.
• A definition on the shorter chain completes without gathering trades or market data and without assembling a bundle.
• A step result is addressed by the name of the step that produced it, and a builder that cannot find or read what it consumes stops the run rather than yielding an empty field.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

Closed without its own change, because PR #2289 delivered the whole of its goal under the sibling story Run a full report: scheduling, reporting and compute, as task Add the run configuration and address phase results by role.

What that change put in place, against this task's acceptance:

• report_definition carries pre_processing, prepared_input_key and post_processing, and the trigger validates the codes and copies them into the run request, so the chain a definition selects is the one that runs.
• One registered definition builds either chain. execute is the seven steps this story's analysis recorded; substitute replaces the four pre-processing steps with resolve_prepared_input, so a run starts from an archive that already exists and gathers nothing.
• Step results moved from positions to names. That went further than this task asked: it removed the positional remap this task's acceptance was written around, and a builder that cannot find or read what it consumes now throws instead of yielding an empty field.

The premise this task was written on, that shortening the chain is a positional remap, is therefore obsolete rather than wrong. The work remaining on the story is the reporting to compute handoff, retrieval, and the ACME fixture; the analysis that established which of those are still needed is Does reporting use what the workflow engine guarantees?.