build: add installable CMake package

[zuudevs]

Summary

Starts v0.9 packaging work by making cpp_request installable and consumable through CMake package discovery.

Packaging

• installs all public include/cpp_request/*.hpp headers
• installs the compiled library
• exports the installed target as cpp_request::cpp_request
• generates and installs cpp_requestConfig.cmake
• generates and installs cpp_requestConfigVersion.cmake
• keeps the existing in-tree cpp_request::internal alias for project tests
• changes the produced library filename to cpp_request

Package verification

Adds an install-tree consumer test that:

1. installs the current build into a clean temporary prefix,
2. configures a completely separate consumer project,
3. resolves the package with find_package(cpp_request CONFIG REQUIRED),
4. links only against cpp_request::cpp_request,
5. builds and runs the consumer executable.

This verifies that build-tree-only include paths and test dependencies do not leak into the installed package.

Build-system cleanup

• uses build/install generator expressions for public include directories
• uses project-local paths instead of top-level CMAKE_SOURCE_DIR assumptions
• corrects stale project metadata that claimed HTTPS and a fluent builder API
• uses pre-1.0 SameMinorVersion package compatibility

Scope

This PR is packaging only. Benchmarks, examples, README/user documentation, and release acceptance remain separate v0.9/v1.0 work.