fix(stata): improve robustness

[Davidvandijcke]

Summary

• S17: Replace unreliable det(XWX) != 0 singularity check with reciprocal condition number (1/cond(XWX) > epsilon(1)) in r3d_locpoly for robust near-singularity detection
• S19: Add user-facing notification when plugin fails and falls back to Mata (4 locations in r3d_bandwidth_select and r3d_estimate_core)
• S20: Fix Gini Lorenz curve endpoint bug in r3d_gini_from_quantile -- prepend 0 instead of qf[1] so the Lorenz curve correctly starts at the origin

Test plan

• Verify r3d_locpoly handles near-singular XWX matrices without crashing (previously could pass det != 0 check but produce garbage from lusolve)
• Verify plugin fallback prints a message (test with plugin unavailable or corrupted)
• Verify Gini coefficient computation matches R implementation for distributions with non-zero minimum quantile values

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