ALICE3: ACTS tracking geometry provider

[pbutti]

Adds the Acts::TrackingGeometry that an ACTS-based reconstruction needs, built from the TGeo geometry O2 already has live in memory. ACTS stays an optional dependency: without it these packages produce no targets.

O2::ACTSInterface (Detectors/ACTS/interface), no DPL dependency:

• TrackingGeometryManager, a lazily-built process-wide provider following the o2::base::Propagator::Instance() / GeometryTGeo::Instance() pattern. It fetches nothing itself; callers make gGeoManager available first, normally by declaring GRPGeomRequest::Aligned.
• ITrackingGeometryBuilder, the detector-specific seam, plus the element store the geometry does not own but depends on.
• SurfaceIndexMap, the O2 sensor <-> ACTS surface lookup.
• MagneticFieldAdapter, an Acts::MagneticFieldProvider over the O2 field.

O2::ACTSWorkflow (Detectors/ACTS/workflow): ActsGeometryService, the same provider reachable through the DPL ServiceRegistry with ServiceKind::DeviceGlobal.

O2::ALICE3ACTS (Detectors/Upgrades/ALICE3/ACTS): the Gen3 Blueprint builder, ported from actsO2 ActsAlgorithms/Geometry/src/ALICE3Gen3Geometry.cpp. The construction logic is unchanged so the geometry identifiers stay compatible with the standalone chain's material maps and digitisation/seeding configs; verified by decorating an O2-built geometry with actsO2's own material map. Differences from the original: it reads a TGeoManager handed in by the caller instead of importing a ROOT file, and its process-lifetime static stores became member state so several builders can coexist in one process.

Sensors are identified by the TGeo node the surface was built from, not by position. A transform is not an identity: the vertex-detector petals are tube segments, so the three layers of a petal share both origin and rotation and differ only in radius, which the O2 matrix cache does not carry. Position-only matching is kept as a fallback and reports such cases as ambiguous instead of assigning them arbitrarily.

macros/run_test.sh generates an ALICE 3 geometry with o2-sim and checks every TRK chip against its ACTS surface; config/gen3_geometry_config.json is the matching geometry description.

@njacazio