[kammce]

Adds mp_units.si, a module with only SI, the ISQ and angular units. mp_units.systems now imports and re-exports it, so all of mp-units stays available, and code that only needs SI can import much less.

Why

With C++ modules, CMake rebuilds the interface of every imported module in each consumer's build. mp_units.systems includes all 16 unit systems and is the slowest of these rebuilds. In libhal's Cortex-M deploys it's rebuilt for every architecture × build type, twice each counting the test_package (libhal/ci#132).

Compiling the module interface with LLVM 23, using the same command CMake generates:

Module contents	Time	BMI size
all 16 systems (mp_units.systems today)	12.3 s	161 MB
SI + ISQ + angular (mp_units.si)	3.7 s	~80 MB

Changes

• New module: src/systems/mp-units-si.cpp defines mp_units.si (angular, isq, isq_angle, si), with CMake target mp-units::si.

• mp_units.systems: does export import mp_units.si; and defines MP_UNITS_SI_MODULE_IMPORTED, instead of including the SI, ISQ and angular headers itself. The two modules can be imported together, because each declaration is attached to only one of them.

  Simply including the SI headers in both modules would break that: Clang rejects it with "declaration 'x' attached to named module 'b' cannot be attached to other modules". I confirmed that with a minimal two-module test.

• Header guards: 18 headers of the other systems (cgs, iau, codata, hep, iec, natural, typographic, yard_pound, astronomy) skip their SI, ISQ and angular includes when MP_UNITS_SI_MODULE_IMPORTED is defined. That follows the existing pattern of skipping framework.h when MP_UNITS_IN_MODULE_INTERFACE is defined. Header-only builds are unchanged.

• Targets: the 26 SI-set headers move from the systems target to the new si target, which systems links. The Conan recipe gets an si component.

Testing

• Local build: conan create with libhal's options (freestanding, cxx_modules, no std_format, no import_std, contracts none, no RTTI) and LLVM 23 passes, including the test_package. The test_package imports the full mp_units umbrella, which brings in mp_units.systems and mp_units.si together.
• Not tested: header-only builds, other compilers, and other option combinations. The fork's CI covers those.

Follow-up

For libhal to benefit, it would import mp_units.core plus mp_units.si and link mp-units::core mp-units::si, instead of mp_units and mp-units::mp-units. That's a separate libhal change.