[Mechanics] Create mechanics library skeleton for phsyicell (not micromechanics/sem)

[marcorusc]

Summary
Scaffold the mechanics (PhysiCell) library target with proper CMake wiring and alias.

Steps

1. Open mechanics/physicell/CMakeLists.txt.
2. Add add_library(physicore.mechanics.physicell) if missing.
3. Add add_library(physicore::mechanics::physicell ALIAS physicore.mechanics.physicell).
4. Use target_sources(... FILE_SET HEADERS ...) to export public headers (will be added in later tasks; can start empty).
5. Link: target_link_libraries(physicore.mechanics.physicell PUBLIC physicore::common).
6. Guard tests with PHYSICORE_BUILD_TESTS; prepare empty tests target (e.g. add_executable(physicell.tests) later).
7. Ensure project builds with cmake --preset=gcc-debug then cmake --build --preset=gcc-debug.

Acceptance Criteria

Configure + build succeeds (gcc-debug preset).
Library target and alias visible in build system.

Verification
Run build presets and confirm target appears in build dir.

Dependencies
None.

Follow-ups
Subsequent issues will populate headers and sources.