Modernize CMake build system with target-based dependencies and FetchContent fallback

[Copilot]

The CMakeLists.txt had accumulated complexity from platform-specific workarounds and used legacy CMake patterns (global variables, FILE(GLOB), include_directories()). This refactors to modern, idiomatic CMake 3.16+.

Core Changes

• Target-based build options: Compile flags encapsulated in Ygor::CompileOptions and Ygor::SanitizerOptions INTERFACE libraries
• Proper transitive dependencies: Ygor::ygor exports include paths and links, downstream projects just target_link_libraries(app PRIVATE Ygor::ygor)
• FetchContent fallback: Eigen auto-downloads if not found locally (controllable via FETCHCONTENT_FALLBACK)
• Graceful degradation: Missing optional dependencies disable themselves with status message instead of failing

CMake Package Export

Installed library now provides proper CMake package config:

find_package(Ygor REQUIRED)
target_link_libraries(myapp PRIVATE Ygor::ygor)

Cross-Compilation Support

Added toolchain files in cmake/toolchains/:

• mingw-w64-x86_64.cmake - Windows
• macos-x86_64.cmake - macOS via osxcross
• aarch64-linux-gnu.cmake / armv7-linux-gnueabihf.cmake - ARM
• musl-linux.cmake - Alpine/embedded

Other Improvements

• Explicit source listing replaces FILE(GLOB)
• Library versioning (SOVERSION)
• Fixed MEMORY_CONSTRAINED_BUILD to preserve Debug assertions
• Generator expressions for config-specific flags

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• gitlab.com
  • Triggering command: /usr/lib/git-core/git-remote-https /usr/lib/git-core/git-remote-https origin REDACTED s-through=-lgcc_/home/REDACTED/work/Ygor/Ygor/src s-th�� d/src --local /x86_64-linux-gnx86_64-linux-gnu ples.cc.o (dns block)
  • Triggering command: /usr/lib/git-core/git-remote-https /usr/lib/git-core/git-remote-https origin REDACTED git conf�� USE_LINUX_SYS=1 --64 --local (dns block)
  • Triggering command: /usr/lib/git-core/git-remote-https /usr/lib/git-core/git-remote-https origin REDACTED make conf�� d/src --local p/bin/as ding CXX object as ect CMakeFiles/c-I (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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[hdclark]

@copilot please review this pull request

[Copilot]

@copilot please review this pull request

I've performed a self-review of this PR. Here's a summary:

Verified working:

• Build succeeds with GCC 13.3 (Release mode)
• Static and shared library builds work correctly
• Installation produces proper CMake package config files
• Downstream find_package(Ygor) works as expected

Key changes:

• Bumped CMake minimum to 3.16 for FetchContent
• INTERFACE libraries encapsulate compile/sanitizer options
• FILE(GLOB) replaced with explicit source listing
• FetchContent fallback for Eigen (gracefully disabled if network unavailable)
• Toolchain files for cross-compilation (MinGW, macOS, ARM, musl)

Areas to verify:

1. The toolchain files are examples and may need adjustment for specific environments
2. FetchContent URL hash for Eigen should be verified if updating to newer versions
3. Some existing platform-specific quirks may need re-testing on those platforms

Build verified in commit df91603.