Clamp bootstrap confidence interval indices

Summary

• Handle NaN adjusted quantiles when bootstrap jackknife variance is zero.
• Clamp calculated confidence-interval indices before indexing the resample.

Root cause

Zero jackknife variance makes the BCa acceleration calculation undefined. Its NaN quantiles were converted to invalid indices, causing an out-of-bounds read.

Testing

• Regression failed before the fix (upper bound was 0.0 instead of 1.0) and passes after with UBSan and Guard Malloc.
• Full UBSan CTest suite passes: 82/82.
• C++14 warning-enabled build passes with -Wall -Wextra -Wshadow -Werror.
• The ASan run stalled and was stopped; it is not counted as a pass.
• Catch2-configured clang-tidy reported no diagnostics on changed lines; a full-file run reported warnings on existing, unchanged code.

Not included

Validation for explicitly configuring zero bootstrap resamples is a separate edge case.