Add spatial performance benchmarks and CI gating

[razinkele]

Files: tests/test_spatial_performance.py and related performance TODOs (e.g., # TODO: Target < 60 seconds for 10 years, 100 patches, 10 groups).

Description: Add deterministic performance benchmarks and fail CI if regression beyond a fixed threshold occurs.

What to do:

• Use pytest-benchmark or a dedicated benchmarks/ folder to track baseline times for common scenarios.
• Add CI job (scheduled or on PR) that runs benchmarks and reports regressions.
• Define acceptable thresholds (e.g., 10-year run, 100 patches, 10 groups < 60s on CI baseline) and make them configurable.

Acceptance criteria:

• Benchmarks run in CI and regressions are reported. Targets are documented and realistic for CI hardware.