Prune case-local BFCL equivalent siblings

[SonAIengine]

Summary

• Suppress model-facing non-priority equivalent siblings when a BFCL case-local surface is present, while keeping retrieval evidence intact.
• Strengthen surface equivalence evidence for currency conversion, definite integral / area-under-curve, Fibonacci sequence / series, and GCD / HCF.
• Add repeated-call prompt guidance and avoid decimal-rate hints for fields explicitly described as percentages.
• Document the xgen-0.27 100-case gate pass and remaining retrieval-layer bottlenecks.

Validation

• poetry run ruff check .
• poetry run ruff format --check .
• poetry run pytest tests/test_bfcl_tool_selection_llm_loop.py tests/test_graphify_contract_025.py tests/test_bfcl_tool_selection_sweep.py -q -> 64 passed
• poetry run pytest tests/ -q -> 967 passed, 3 skipped, 1 warning
• Fresh Qwen hardcase run: /tmp/gtc-bfcl-equivalent-sibling-pruning-hardcases-v2.json -> 6 / 10 pass
• Fresh Qwen 100-case sweep: /tmp/gtc-bfcl-equivalent-sibling-pruning-limit25-sweep.json -> retrieved exact 0.96, retrieval@5 0.99, row preservation 0.98, parallel_multiple exact 1.00, xgen-0.27 gate pass

Remaining Bottlenecks

• multiple_24: route retrieval miss; expected route_planner.calculate_route is not in top-5.
• parallel_3: over-decomposition where the model calls lower-level sequence-only and sequence+3D tools together.

[SonAIengine]

Closing this internal stacked PR as superseded by the current main branch. The branch is preserved for history.