refactor: route planner type lookup through resolver

[ktjn]

Summary

• route planner enum and named-type lookup through the common declaration resolver
• preserve exact-version handling, semantic-only filtering, model fallback, and plan output

Validation

• uv run ruff format .
• uv run ruff check .
• uv run python ../.github/scripts/check_mypy_baseline.py --baseline mypy-baseline.txt -- uv run mypy src/modelable --no-error-summary --show-error-codes
• planner/browser/semantic focused tests (18 passed)
• full suite serially: 3155 passed, 67 skipped

Reviewed by subagent: no findings.