Add structural archetype truss benchmarks (cantilever, Pratt, Warren variants)

[cmccomb]

Summary

Add explicit truss benchmark archetypes that are currently missing or underrepresented in the packaged catalog, using DesignBench artifacts as seeds.

Proposed archetypes

• Cantilever truss (from DesignBench cantilever_01 style definition).
• Pratt-like bridge truss (from bridge_pratt_03 style data).
• Warren bridge variants (from bridge_warren_01 and bridge_simple variant families).

Proposed scope

• Add catalog entries for each archetype as first-class problems with stable IDs.
• Normalize support conditions, load definitions, and objective/constraint fields to this repository's schemas.
• Ensure each archetype has at least one well-defined baseline configuration and expected behavior.
• Add examples that demonstrate loading and solving/evaluating each archetype.
• Add tests that validate:
  • parsing/registry loading,
  • structural analysis execution,
  • objective/constraint outputs are finite and deterministic.

Notes from source material

• Candidate seeds exist in DesignBench under:
  • data/problems/cantilever.json
  • data/modification_trees/bridge_pratt_03_tree.json
  • data/modification_trees/bridge_warren_01_tree.json
  • data/modification_trees/bridge_simple_01_variant*.json

Out of scope

• Eval split generation and split-specific metadata.

Acceptance criteria

• New archetype problems are discoverable via the registry and documented in generated catalog docs.
• At least one example per archetype runs end-to-end.
• Tests cover at least one invariant per archetype (e.g., support condition sanity, feasible analysis execution, bounded outputs).