Installed package does not compile for a consumer: tmech headers are neither installed nor exported

[petlenz]

With the default configuration (tmech fetched, NUMSIM_USE_SYSTEM_TMECH=OFF), cmake --install followed by a consumer doing

find_package(numsim-materials REQUIRED)
target_link_libraries(app PRIVATE numsim-materials::numsim-materials)

configures successfully and then fails to compile:

error: 'mandel' is not a member of 'tmech'

CMakeLists.txt:128-134 adds the fetched tmech include directory to the interface without linking the target, to keep it out of the export set. That directory is a build-tree path, so the installed package either points at nothing or picks up an older system tmech.

Fix (either)

• install the fetched tmech headers alongside ours and add the installed path to the interface via $<INSTALL_INTERFACE:...>, or
• require an installed tmech for an installable build and export the dependency, with a version check once tmech's config provides one.

Test

A CI job that installs to a scratch prefix and builds a one-file consumer against it.

All references are to main at 91bd26f.