feat(build): default BUILD_SHARED_LIBS to ON

[olantwin]

Summary

• Default `BUILD_SHARED_LIBS` to `ON` via `option()` at the top of the top-level `CMakeLists.txt`.

Motivation

The main `SHiPGeometry` library and every subsystem (`Cavern`, `Target`, `MuonShield`, `Magnet`, `DecayVolume`, `Trackers`, `Calorimeter`, `UpstreamTagger`, `TimingDetector`) use bare `add_library( src/...)` — no `SHARED` keyword. Without setting `BUILD_SHARED_LIBS=ON` in the configure, CMake produces static archives in `lib/` instead of the `.so` files the rest of the SHiP stack (`SHiPGeometryService`, `aegir`, conda packagers) consumes.

Surfaced while writing a conda recipe for `SHiPGeometry` — the recipe initially silently produced `.a` files and the downstream tests failed. Default to `ON` so the shared library this project clearly intends to provide is what you get out of the box; users who want static can still pass `-DBUILD_SHARED_LIBS=OFF` explicitly.

Test plan

• CI (`build-test.yml`) stays green.
• Manual: after this change, `cmake -S . -B /tmp/g` produces `libSHiPGeometry.so` and `libCavern.so` etc. in the build tree (rather than `.a`).

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: ffbad629-6126-4a3a-8b9f-92227e4dff72

📥 Commits

Reviewing files that changed from the base of the PR and between a4df127 and 06c5c05.

📒 Files selected for processing (1)

• CMakeLists.txt

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/default-shared-libs

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