Lifted linear symmetries

The action of G̃L⁺(2, ℝ) on slicings, pre-stability conditions and stability conditions, and its continuity.

The group itself -- the compatible-pair construction, its deck transformations, the covering map, simple connectedness and the topological-group laws -- is not here. It is neutral mathematics and lives beside the general-linear-group API, under LinearAlgebra/Matrix/GeneralLinearGroup/UniversalCover/, with the +1-equivariant order automorphisms of ℝ under Algebra/Order/NormalizedShift/ and the general product-of-coverings lemma under Topology/Covering/. MO1.12 (#1323), cutover-ledger row 08.