Default MSVC static CRT for static yaml-cpp builds

[harleywilsoneng]

Fixes #1354

Summary

• Default YAML_MSVC_SHARED_RT to ${YAML_BUILD_SHARED_LIBS} so static yaml-cpp builds use /MT by default on MSVC.
• Document static CRT behavior and YAML_CPP_STATIC_DEFINE for consumers linking the static library.

Test plan

• Configure a static MSVC build and verify /MT is selected by default.
• Configure with -DYAML_MSVC_SHARED_RT=ON and verify /MD is still available.

[SGSSGene]

Amazing! This is highly appreciated!

I took the liberty to convert it to a "draft" PR. Please convert it back to a normal PR when it is ready to be reviewed.

[harleywilsoneng]

Thanks @SGSSGene — marking this ready for review.

CI is green on the latest push (including Windows static consumer CRT alignment in test/cmake/CMakeLists.txt). Happy to adjust anything else if you'd like changes.

[harleywilsoneng]

Thanks @SGSSGene — marking this ready for review.

CI is green on the latest push (including Windows static consumer CRT alignment in test/cmake/CMakeLists.txt). Happy to adjust anything else if you'd like changes.

[SGSSGene]

Thank you for tackling this issue!
I only have a small remark about the README. Please feel free to reformulate my suggestion it in any other way!

[SGSSGene]

I would rewrite this slightly, It sounds like by default everyone should define YAML_CPP_STATIC_DEFINE.
I'd rather recommend consume target yaml-cpp::yaml-cpp and have the information about YAML_CPP_STATIC_DEFINE as a side note.

Suggested change

	the dynamic CRT (`/MD`) instead. When linking yaml-cpp statically into your project, define
	`YAML_CPP_STATIC_DEFINE` in your target (or consume the `yaml-cpp::yaml-cpp` imported target,
	which sets it automatically).
	the dynamic CRT (`/MD`) instead. When using cmake consume `yaml-cpp::yaml-cpp`, e.g.: `target_link_libraries(your_project_target yaml::yaml`.
	(If cmake is not used you must define `YAML_CPP_STATIC_DEFINE` for your target.)

[harleywilsoneng]

Good catch — updated the README to lead with yaml-cpp::yaml-cpp, and mention YAML_CPP_STATIC_DEFINE only as the non-CMake fallback.

[SGSSGene]

Nice work! thanks!