Update main to 3.x

[madbaron]

This PR implements the changes to start building releases with fully gaudi-based reconstruction workflows

[Copilot]

Pull Request Overview

This PR transitions the MuonCollider software stack from Marlin-based to Gaudi-based reconstruction workflows, marking a significant architectural change in the codebase. The changes include replacing legacy ILCSoft packages with modern Key4hep/Gaudi equivalents and updating the build infrastructure.

• Removes 7 Marlin-based reconstruction packages (overlay, lctuple, marlintrkprocessors, forwardtracking, ddmarlinpandora, conformaltracking, actstracking)
• Adds 3 new Gaudi-based packages (k4reco, k4gaudipandora, k4actstracking) to replace Marlin functionality
• Updates mucoll-stack to version 3.0 with new dependencies including ML tools (py-torch, acorn) and reorganized package dependencies
• Modifies Docker and CI/CD configurations to support the new Gaudi-based workflow

Reviewed Changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
packages/overlay/package.py	Removed Marlin-based event overlay package
packages/marlintrkprocessors/package.py	Removed Marlin tracking processors package
packages/lctuple/package.py	Removed Marlin ROOT tuple creation package
packages/forwardtracking/package.py	Removed Marlin forward tracking package
packages/forwardtracking/testing.patch	Removed associated test patch for forwardtracking
packages/ddmarlinpandora/package.py	Removed Marlin-PandoraPFA interface package
packages/conformaltracking/package.py	Removed Marlin conformal tracking package
packages/actstracking/package.py	Removed Marlin ACTS tracking integration
packages/k4reco/package.py	Added new Gaudi-based reconstruction algorithms package
packages/k4gaudipandora/package.py	Added new Gaudi-PandoraPFA interface
packages/k4actstracking/package.py	Added new Key4hep ACTS tracking package
packages/mucoll-stack/package.py	Updated to version 3.0 with new dependencies, environment variables, and pytorch CMAKE_PREFIX_PATH configuration
environments/mucoll-release-ml/spack.yaml	Added gcc externals configuration and reformatted variants
environments/mucoll-common/packages.yaml	Updated package requirements to use Gaudi-based packages and removed Marlin-based ones; updated ACTS to version 44.1.0
Docker/Dockerfile-sim	Changed base image to latest tag and modified entrypoint to auto-source setup script
Docker/Dockerfile-ml	Added mucoll-spack repo update with hardcoded branch references
.github/workflows/concretize.yaml	Restricted CI to ubuntu24 only, removed alma9 and release-ml targets
.github/workflows/build.yaml	Restricted builds to ubuntu24 only
.cherry-pick	Added git cherry-pick commands for specific commits

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