fix: install apps/ executables

[olantwin]

Summary

• Add an `install(TARGETS ...)` rule for `build_geometry` and `validate_geometry` in `apps/CMakeLists.txt` so the executables actually land in the install tree.
• Pull `GNUInstallDirs` in locally — the top-level file's `include(GNUInstallDirs)` happens later, after `add_subdirectory(apps)` returns.

Motivation

`build_geometry` and `validate_geometry` are the project's primary CLI tools but were never installed: `apps/CMakeLists.txt` defined them and stopped there, and the top-level `install(TARGETS ...)` block covers only the library targets. Packaged builds (e.g. our conda recipe) silently lost both binaries, and downstream users had no way to invoke them except by reaching into the build tree.

Test plan

• CI (`build-test.yml`) stays green — the change is purely additive.
• Manual: `cmake -S . -B /tmp/g && cmake --build /tmp/g && cmake --install /tmp/g --prefix /tmp/g-install` produces `/tmp/g-install/bin/build_geometry` and `/tmp/g-install/bin/validate_geometry`.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: eb798ecf-cf59-41f3-a34d-b134350bfb44

📥 Commits

Reviewing files that changed from the base of the PR and between a5bac91 and 4a6f24b.

📒 Files selected for processing (1)

• apps/CMakeLists.txt

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/install-apps-executables

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