Remove the unused economy comparison module

[anth-volk]

Fixes #3935

Summary

Stacked on #3934.

policyengine_api/services/economy_comparison.py has had no caller in the package since #2569, which removed the Public API's own society-wide simulations. This PR removes the module with its unit tests, its snapshot tests and its snapshot data: 915 lines of code, about 1,600 lines of tests and an 8,000-line JSON file.

• No production behaviour changes. Nothing in policyengine_api imports the module. The only other matches for economy_comparison are the economy_comparison report kind in report_spec_service.py, which is a string, and a docstring.
• The enhanced FRS weight-matrix dependency goes with it. The module's uk_constituency_breakdown and uk_local_authority_breakdown downloaded the enhanced FRS constituency and local-authority weight matrices. UK: certify a Microcosm national dataset as the UK default, with separate national and local-area datasets and a corrected local-authority list policyengine.py#553 lists that as a Public API dependency to resolve before a Microcosm UK default.
• Rules:
  • api.routes.no-domain-logic-in-http drops test_economy_comparison_logic_is_not_in_the_http_layer, which only checked that the module stayed out of the HTTP layer.
  • The rule keeps its calculation-route check, and its statement now reads "Flask routes delegate domain processing, such as calculation, to services instead of doing it in the HTTP layer."
  • AGENTS.md and docs/generated/rules.md are re-rendered.
• Docstring: the congressional district metadata model's docstring pointed at the module as an example of Pydantic models. It now refers to the codebase's other Pydantic models.
• Left for later: microdf_python stays declared, although nothing imports microdf any more. Removing an unused dependency is a separate cleanup; step 2.7's dependency check would flag it.

The weighted-microdata rules (step 1.9) are the next PR in the stack. With this module gone, they start enforced with no findings.

Testing

• pytest tests/unit: 2,727 passed. The 97 removed tests are exactly the module's unit and snapshot tests, plus the one route-boundary test.
• uv run --frozen --extra dev mypy: no issues. The module was outside the type-check scope, so the file count is unchanged.
• python scripts/run_quality_guards.py: migration-contracts, rules-registry and ci-scripts passed.
• python -m scripts.rules render leaves no diff, and python -m scripts.rules guard passes.
• ruff check . and ruff format --check . (ruff 0.15.12, from uv.lock) are clean.

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