Native CMake integration

[gabrielecastellano]

This creates a root CMakeLists that can be used to directly configure and build the project natively.

For instance:

$ mkdir cmake-cache && cd cmake-cache
$ cmake -DBUILD_SIM=ON -G "Unix Makefiles" ..
$ cmake --build . -j 103

This can be helpful for developing (code inspection features) and for future maintenance in general.

Works with PR aliyun/ns-3-alibabacloud#9 for the ns3 subproject.

(tested with simulator only)

[Copilot]

Pull request overview

Adds a top-level CMake entrypoint to configure/build SimAI “natively” (without the existing wrapper scripts), primarily targeting the ns-3 simulator workflow and integrating Astra-sim sources into the ns-3 tree during configuration.

Changes:

• Introduces a new root CMakeLists.txt to drive builds of the simulator (and optionally analytical/phy targets).
• Adjusts astra_ns3 subproject CMake to only default CMAKE_BUILD_TYPE when unset.
• Updates .gitignore to ignore additional build/config artifacts (including generic **/build and **/cmake-cache).

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 5 comments.

File	Description
CMakeLists.txt	New root CMake configuration orchestrating Astra-sim + ns-3 build and creating a simulator symlink
astra-sim-alibabacloud/build/astra_ns3/CMakeLists.txt	Safer defaulting of CMAKE_BUILD_TYPE (doesn’t clobber user-provided value)
.gitignore	Adds ignores for config/build artifacts and introduces broad **/build / **/cmake-cache patterns

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[gabrielecastellano]

I have merged latest changes from master and implemented comments from copilot.