Support independent nested installs and Windows DLL consumers

[sean-parent]

Summary

• Add optional INSTALL_OPTION and target-scoped deferred install metadata, allowing independent same-namespace package installation.
• Preserve installed transitive linkage between separately installed nested packages.
• Keep compiled library type under consumer control through BUILD_SHARED_LIBS (static by default); no explicit library-type override is introduced.
• Automatically deploy runtime DLL dependencies beside toolkit-created test/example executables on Windows, with CMake 3.24-compatible handling of empty lists and already-adjacent DLLs.
• Add Windows static/shared installed-consumer CI coverage using an explicitly exported, versioned C ABI leaf and transitive parent package.
• Document stlab-execution's explicit .def export pattern, ABI-boundary responsibilities, and downstream application deployment ownership.

This enables the standalone stlab/stlab-execution extraction and STLab's transitive dependency.

Validation

• All 11 nested configure/build/install/consumer cases passed on Windows with CMake 3.24.4 and 4.3.1, including shared/static consumers, independently staged test/example DLLs, paths containing spaces, empty DLL lists, and already-adjacent DLLs.
• All 4 target-type configure cases passed with both CMake versions: compiled static/shared and header-only defaults, with the parent BUILD_SHARED_LIBS setting preserved.
• The new shared-consumer regression failed before deployment was implemented because the installed DLL was absent beside the executable.
• Existing dependency/provider/version baselines passed (32 dependency cases passed, one existing case skipped).
• Hosted Ubuntu/Windows checks run separately from these local results.

No release is created by this PR. Merge/release this support before publishing execution, then STLab. The execution companion needs its corresponding configuration and pin update for the standard BUILD_SHARED_LIBS policy; that repository is not modified here.

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Copilot review overview

🟡 Changes recommended

The installed parent target drops its transitive leaf linkage because the dependency is build-interface-only.

Review effort: Balanced
Findings: 1

Open (1)

• Preserve stlab::leaf in the installed stlab::parent interface · New

What changed in this PR

Adds explicit compiled-library types and package-specific install controls for nested libraries.

Changes:

• Adds LIBRARY_TYPE STATIC|SHARED and INSTALL_OPTION.
• Scopes deferred installation metadata per target.
• Adds nested-install and target-type regressions to CI.

File	Description
cpp-library.cmake	Parses and forwards new options.
cmake/​cpp-library-setup.cmake	Creates explicit static/shared targets.
cmake/​cpp-library-install.cmake	Adds independent install options and target-scoped metadata.
README.md	Documents new APIs and regressions.
.github/​workflows/​ci.yml	Runs added tests.
tests/​setup/​test_target_type.cmake	Tests target-type behavior and diagnostics.
tests/​setup/​fixtures/​target_type/​CMakeLists.txt	Configures target-type cases.
tests/​setup/​fixtures/​target_type/​src/​sample.cpp	Provides compiled fixture source.
tests/​setup/​fixtures/​target_type/​include/​fixture/​sample.hpp	Provides fixture API.
tests/​install/​test_nested_install.cmake	Exercises nested installation scenarios.
tests/​install/​fixtures/​nested/​CMakeLists.txt	Defines the parent package and dependency.
tests/​install/​fixtures/​nested/​include/​stlab/​parent.hpp	Provides parent fixture API.
tests/​install/​fixtures/​nested/​leaf/​CMakeLists.txt	Defines the nested leaf package.
tests/​install/​fixtures/​nested/​leaf/​include/​stlab/​leaf.hpp	Provides leaf fixture API.
tests/​install/​fixtures/​consumer/​CMakeLists.txt	Configures the installed-package consumer.
tests/​install/​fixtures/​consumer/​main.cpp	Exercises installed fixture APIs.

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