Upgrade simulation API to PolicyEngine v4.10

[anth-volk]

Context

policyengine 4.10.0 now includes the output support needed by the simulation API, including UK geography asset resolution. The simulation API should move onto this runtime while preserving existing public API contracts.

Scope

• Pin the simulation runtime to policyengine==4.10.0.
• Keep the existing single-year macro result shape for API callers.
• Keep the compact budget-window response contract.
• Adapt PolicyEngine v4 output objects back into the API v2 legacy macro output dictionary.
• Wire UK constituency and local authority outputs into the existing result keys.
• Add AI/PR guidance so future non-trivial work opens an issue first and uses draft PRs with Fixes #ISSUE_NUMBER at the top of the description.

Testing

Add focused unit and contract coverage for dependency pinning, output adaptation, API metadata, and existing result schema contracts.