Fix the executor simulation runtime's 54 Pyright errors #870

Problem

projects/policyengine-simulation-executor/src/policyengine_simulation_executor/simulation_runtime.py, the annual simulation's runtime, has 54 Pyright errors, tolerated by rules/baselines/pyright-executor.txt (sim.quality.pyright-executor, #864):

• 47 possibly-unbound variables, each assigned inside with runtime.span(...), which Pyright treats as possibly swallowing the exception because policyengine-observability types its scope managers' __exit__ as returning bool;
• 2 argument errors that follow from those (Unbound | Simulation);
• 5 others: the compiled reform possibly None, the UK region type, the input dataset's entity_data, and a parameter reassigned with a wider type.

Proposal

Fix all 54 without changing what a run does: open spans with scopes.span (#864), correct _build_simulation's return annotation, and narrow the other values where they're read. The executor's baseline shrinks by 54.