fix(router): honor explicit stats baseline

[snvtac]

Summary

• Prefer an explicit router.stats.baselineModel when constructing router stats.
• Keep router.scenarios.default as the fallback baseline when no explicit stats baseline is configured.
• Add regression coverage for explicit-over-default, default fallback, and explicit baseline without a default scenario.

Fixes #250

Testing

• npm test
• npm exec tsc -- -p tsconfig.json --noEmit