Add interactive installation script and README

scripts/installation/README.md

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+# Installation Scripts
+
+Interactive installer for [REST-for-Physics](https://github.com/rest-for-physics/framework) and its dependencies.
+
+## Quick start
+
+```bash
+bash install_rest.sh
+```
+
+The script is interactive and will guide you through each step.
+
+## Install modes
+
+### Mode 1 -- Server install
+
+For machines where ROOT, Geant4 and Garfield++ are already available system-wide
+(e.g. the `titan` cluster at Zaragoza).
+
+You will be asked for the path to the system `thisREST.sh` that loads the
+pre-installed environment. The script then clones, configures and builds only
+REST itself.
+
+### Mode 2 -- Full install from source
+
+For a fresh Ubuntu (22.04/24.04) or WSL system with nothing pre-installed.
+Builds everything from source:
+
+| Package       | Version                          |
+|---------------|----------------------------------|
+| ROOT          | 6.26.10                          |
+| Geant4        | 11.0.3 (with GDML + data)       |
+| Garfield++    | pinned commit `a993c5eb`         |
+| REST          | latest from `master`             |
+
+System dependencies are installed via `apt` (requires `sudo`).
+If `sudo` is not available the script offers to skip this step, assuming
+packages are already present.
+
+## What the script does
+
+1. Installs system packages (mode 2, optional)
+2. Downloads and builds ROOT, Geant4, Garfield++ (mode 2)
+3. Asks you to choose which REST libraries to enable
+4. Clones the REST framework and pulls submodules
+5. Configures and builds REST with CMake
+6. Tests the installation (`restRoot`)
+7. Optionally adds the environment setup to `~/.bashrc`
+
+## Options asked during installation
+
+- **Install directory** -- where REST source and build will live (default: `~/rest`)
+- **Libraries** -- all, a common subset, or pick individually
+- **Garfield++ / Geant4 integration** -- enable or disable
+- **Submodule strategy** -- latest commits or framework-pinned versions
+- **Parallel jobs** -- capped at 16 for shared-server politeness
+
+## Individual scripts (legacy)
+
+The older per-package scripts are still available for reference:
+
+- `installROOT.sh` -- ROOT 6.28.02
+- `installGeant4.sh` -- Geant4 11.0.3
+- `installGarfield.sh` -- Garfield++
+- `installRequiredSoftware.sh` -- system packages via `apt-get`
+
+`install_rest.sh` supersedes all of them in a single guided workflow.
+
+## Requirements
+
+- **OS**: Ubuntu 22.04 / 24.04 (or WSL with either). Other Linux distros may
+  work but the `apt` package list would need adapting.
+- **Disk**: ~5 GB for a full build (ROOT + Geant4 + Garfield + REST)
+- **Tools**: `bash`, `git`, `python3`, `cmake`, `make`, `wget`