Issue fixes: monotonic durations, installable SDK, static builds with libcurl, system nlohmann/json

[turtledreams]

What

Fixes for four open issues:

• Calculating durations should use a monotonic clock instead of system_clock #100: session, view and timed event durations use std::chrono::steady_clock, so a system clock change no longer distorts them. Request timestamps stay on system_clock.
• Using -DCMAKE_INSTALL_PREFIX with build produces flat include folder & does not include DLL #103: cmake --install puts countly.hpp in the include root, all public headers under include/countly, and the bundled nlohmann/json next to them. The missing DLL from the same issue was already fixed in 26.8.0.
• why does the CMakeList check to ensure you provide a custom HTTPClient callback when building as a static library? #36: static builds on Linux and macOS can use the built-in HTTP client (libcurl); the configure-time error is gone.
• third-party JSON dependency included in header #34: new COUNTLY_USE_SYSTEM_JSON CMake option (default OFF) builds the SDK against the app's own nlohmann/json 3.11+. The headers now stop with a clear error when they find a nlohmann/json older than 3.11 first on the include path. The CHANGELOG marks that as a minor breaking change: such builds compiled before, while mixing two versions of the library.

Tidy (second commit)

• CHANGELOG: section renamed to ## XX.XX.XX, one line per entry in the file's style (the long lines failed Codacy's MD013), and the breaking line above added.
• Comments cut to single-line notes.
• The nlohmann/json version error now names the COUNTLY_USE_SYSTEM_JSON remedy.
• CI: static-builtin-http and system-json flavors, and an install job (shared and static) that compiles and runs an app against the installed prefix only.

Verification

Local runs on Ubuntu 20.04 (WSL, g++ 9.4, CMake 3.16) and Windows (MSVC 2026), staging against this branch:

Check	staging	this branch
Static build with the built-in HTTP client (Linux)	configure error	builds and installs; the app links and runs
App compiled against the installed tree only (Linux shared, Linux static, Windows DLL)	countly.hpp: No such file	compiles and runs
App that finds nlohmann/json 3.7.3 first on its include path	compiles, mixing 3.7.3 with 3.12.0	stops with the new error
COUNTLY_USE_SYSTEM_JSON=ON with the system nlohmann/json 3.7.3	-	configure error (requires 3.11)
COUNTLY_USE_SYSTEM_JSON=ON with a nlohmann/json 3.12.0 package	-	builds, unit tests pass (68/68); the install skips the bundled JSON

The new CI configurations (install-shared, install-static, static-builtin-http, system-json) also pass locally. In one system-json run, the existing test "Tests setting 'setEventsToRQThreshold' before we start the SDK" found the HTTP queue empty once; it then passed 5 of 5 times on its own and in a full rerun (83/83), so it looks timing-dependent rather than related to this change.

Not covered by a test: a system clock change during a session. The durations read steady_clock::now() directly, so there is no seam to fake the clock; reviewed in the code.

Notes for docs

• An app that links the installed static library also links libcurl and OpenSSL (-lcurl -lssl -lcrypto -lpthread). Builds that add the SDK with add_subdirectory get these from CMake.

Addresses #100, #103, #36, #34.

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https://claude.ai/code/session_017YYaqXRqRFW3PxHyzgwNa9