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Summary

• Add generated mappings for libm::lgamma_r and libm::lgammaf_r, bringing coverage to 110 of 124 f32/f64 exports.
• Compose libm's (log(abs(gamma(x))), signgam) tuple from the existing validated libdevice lgamma, floor, and fmod symbols. Handle negative intervals, integer poles, signed zero, and infinities.
• Add compiler tests for both backends, tuple layouts, and invalid signatures, plus a CUDA example comparing both precisions against libm.

Validation

• cargo run -p cuda-intrinsics-gen --locked -- check
• cargo test -p cuda-intrinsics-gen -p dialect-mir -p mir-importer -p mir-lower --all-targets --locked
• cargo clippy -p cuda-intrinsics-gen -p dialect-mir -p mir-importer -p mir-lower --all-targets --locked -- -D warnings
• cargo oxide run math_lgamma_r: 208 checks passed across f32 and f64
• just check: passed

Depends on #1651. Part of #1224; the remaining 14 exports are outside this PR.

Testing

• just -f cuda-oxide/Justfile check passes (the local mirror of CI: fmt, clippy, tests, guards, docs)
• cargo oxide run <example> passes, or cuda-oxide/scripts/smoketest.sh -o '^<example>$'
• New example added

Checklist

• All commits signed off (git commit -s)
• SPDX headers on new source files

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