[Sophia Wen (hfwen0502)]

Each extension module is one large compile of src/sbd/bindings.cpp, and setup.py build_ext built them one at a time, leaving the build host nearly idle. They now compile in parallel.

• SBD_BUILD_JOBS sets how many modules compile at once. Leave it unset to build all of them in parallel, capped by the CPU count. Set a smaller number, or 1, on a build host or container short of memory: a Thrust module's compile alone peaks near 4 GB. An explicit build_ext -j N still takes precedence, and an invalid value stops the build with an error.
• Each module builds in its own temp directory. Every module compiles the same source file with different defines, and setuptools would place all their objects at one path. In parallel they would overwrite each other, and a module could silently link another's code, such as a UHF module without -D_UHF.
• tox.ini passes SBD_BUILD_JOBS through, and INSTALL.md documents it. CI is unchanged and uses the default.

Build time

Host	Modules	Serial (SBD_BUILD_JOBS=1)	Parallel (default)
8× H100 server, NVHPC	5 (CPU, Thrust and OpenMP offload, plus the UHF builds)	617 s	166 s
macOS laptop, clang	2 (CPU and CPU UHF)	24 s	13 s

Checked

• Each module built in parallel reports the flags of its own build (__sbd_uhf__, __sbd_offload_target__, __sbd_has_evaluator__, __sbd_has_caop__), so none linked another's object.
• The single-process test suite passes on the cpu, gpu and gpu-omp backends built in parallel.
• An invalid SBD_BUILD_JOBS (0, x) stops the build with an error.

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