refactor: share Oscar-independent VGIT predicates

[pbelmans]

Summary

• centralize the cached equations and inequalities defining semistable cones and VGIT walls in core QuiverTools
• provide exact Oscar-independent membership, chamber, and chamber-closure predicates
• move git_equivalent out of the Oscar extension
• make Oscar’s sst and cone construction consume the shared data
• test the core predicates before Oscar is loaded and compare them against Oscar polyhedra afterward

This PR is stacked on #33. The nullcone and wall-projection PR #48 will be rebased onto it.

Tests

• Pkg.test()
• 138 focused core/Oscar equivalence checks