Add relaxed Vector DB ANN task

[joyemang33]

Summary

• Add vector_db_ann_relaxed, a new Frontier-CS 2.0 task variant of Vector DB ANN.
• Keep the same SIFT1M-scale service contract, resource budget, concurrency, and recall target.
• Relax the load/index-build penalty from 0.01 to 0.001, making stronger offline indexing strategies more viable.
• Use a separate relaxed cache namespace and penalty-sensitive baseline metadata filename to avoid cross-task baseline cache contamination.
• Update the 2.0 problem count and 2.0 README.

Please read CONTRIBUTING.md before submitting.

Type of Change

• New research problem
• New algorithmic problem
• New Frontier-CS 2.0 problem
• Bug fix
• Documentation update
• Other:

Testing

• python3 -m py_compile 2.0/problems/vector_db_ann_relaxed/evaluator.py
• git diff --check
• uv run frontier list 2.0
• uv run frontier show 2.0 vector_db_ann_relaxed
• Generated Harbor task with frontier_cs_2_0.main --task-ids vector_db_ann_relaxed
• Verified generated evaluator includes load_penalty_weight=0.001 and relaxed cache namespace.

Checklist

• Code follows the project structure and conventions
• Self-review completed
• Documentation updated (if applicable)

CI Validation (for new problems)

This adds a new Frontier-CS 2.0 task. It includes reference.py; the project-style Rust skeleton and reference.rs are included for validator/reference coverage.