refactor(Calorimeter): obtain tomlplusplus via find_package

[olantwin]

Summary

• Replace the `FetchContent_Declare(tomlplusplus)` block in `subsystems/Calorimeter/CMakeLists.txt` with a single `find_package(tomlplusplus REQUIRED)` call.
• Aligns with how the repo treats its other production dependencies (`GeoModelCore`, `GeoModelIO`, `GeoModelTools` — all `find_package(... REQUIRED)` in the top-level `CMakeLists.txt`).
• The `$<BUILD_INTERFACE:tomlplusplus::tomlplusplus>` wrapping in `target_link_libraries` is preserved — that's an install-time export concern and is independent of how the package is resolved.

Motivation

Surfaced while writing a `shipgeometry` recipe for `ship-conda-recipes`. `FetchContent` requires network at configure time, which we'd rather avoid in packaged builds. `tomlplusplus` is available on conda-forge (and as e.g. `libtomlplusplus-dev` on Debian/Ubuntu), so requiring it as a system dep is a small environment-setup bump rather than real friction.

Behaviour change

Builds without `tomlplusplus` installed will now fail at configure time with a clear CMake error pointing at the missing package, rather than silently downloading from GitHub. For contributors, `pixi install` / `apt install libtomlplusplus-dev` / `brew install tomlplusplus` covers it.

Test plan

• CI (`build-test.yml`) stays green — the LCG / CVMFS environment should already provide `tomlplusplus`.
• Existing Catch2 tests in `subsystems/Calorimeter/test_calorimeter.cpp` continue to pass (they exercise CalorimeterConfig's toml++ parsing).

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 609aa986-2356-47e4-9882-18589e6ce885

📥 Commits

Reviewing files that changed from the base of the PR and between a4df127 and 140d6d0.

📒 Files selected for processing (1)

• subsystems/Calorimeter/CMakeLists.txt

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch refactor/calorimeter-tomlplusplus-find-package

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