fix: add STDEXEC_BUILD_SYSTEM_CONTEXT option and fix Conan recipe for header-only vs compiled package

[solarispika]

Fixes #1540

Problem

The Conan recipe had two related issues:

1. CMake always built system_context — there was no option to skip it, even though the README says the library is header-only.
2. package_id() called self.info.clear() unconditionally — this treated the package as always header-only, even when system_context (a compiled binary) was included. The cpp_info.libs = ["system_context"] entry was also generating an incorrect/missing CMake target.

Changes

CMake (CMakeLists.txt, test/CMakeLists.txt)

• Added STDEXEC_BUILD_SYSTEM_CONTEXT option (default OFF) to match the README's header-only claim
• Wrapped system_context library target creation and installation in if(STDEXEC_BUILD_SYSTEM_CONTEXT)
• Wrapped test.system_context_replaceability test target in the same guard

Conan (conanfile.py)

• Added system_context option (default False)
• Added configure() to set package_type dynamically ("header-library" or "static-library")
• Fixed package_id() to only clear settings when system_context=False (i.e. header-only)
• Fixed package_info() to declare system_context as a component so CMakeDeps correctly generates the STDEXEC::system_context CMake target

Test package (test_package/CMakeLists.txt, test_package/test.cpp)

• Use $<TARGET_NAME_IF_EXISTS:STDEXEC::system_context> to link conditionally
• Use if(TARGET STDEXEC::system_context) to set STDEXEC_TEST_SYSTEM_CONTEXT compile definition
• test.cpp selects between exec::get_parallel_scheduler() and exec::static_thread_pool at compile time via #ifdef

Testing

# Header-only (default) — package_id is empty hash (da39a3ee...)
conan create . --build=missing -tf test_package

# With system_context — package_id reflects platform/compiler settings
conan create . -o system_context=True --build=missing -tf test_package

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